Revert "Add a prefix for objc category."

This reverts commit 181ea6e414c5982015ce161e6368120be3658ec4.

Reason for revert: Breaks downstream project. Kári will help to land it next week.

Original change's description:
> Add a prefix for objc category.
>
> According to the Google Objective-C style [1], category names should
> start with an appropriate prefix. WebRTC has some category definitions
> for system interfaces, but it doesn't use prefixes.
>
> $ otool -ov WebRTC.framework/WebRTC | grep -E "^[0-9a-z]{16} 0x[0-9a-z]+ __OBJC_._CATEGORY" | grep -v "_RTC"
> 0000000002160840 0x217c3c0 __OBJC_$_CATEGORY_UIDevice_$_H264Profile
> 0000000002160850 0x21808b8 __OBJC_$_CATEGORY_AVCaptureSession_$_DevicePosition
> 0000000002160858 0x2180968 __OBJC_$_CATEGORY_NSString_$_StdString
> 0000000002160860 0x21809c8 __OBJC_$_CATEGORY_NSString_$_AbslStringView
>
> To avoid conflicts, prefix the names and methods of those categories.
> Also remove sdk/objc/Framework/Classes/Common/NSString+StdString.h as
> it is not used by any other files.
>
> [1] https://google.github.io/styleguide/objcguide.html#category-naming
>
> Bug: webrtc:13884
> Change-Id: I2cf2742af198ab4e0bfb15c0476d72971e50ceee
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/262341
> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
> Commit-Queue: Daniel.L (Byoungchan) Lee <daniel.l@hpcnt.com>
> Reviewed-by: Artem Titov <titovartem@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#36880}

Bug: webrtc:13884
Change-Id: I85257088e4a3a62e01ff925ab5e77af83b078ef3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/262420
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Auto-Submit: Artem Titov <titovartem@webrtc.org>
Owners-Override: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36885}
diff --git a/sdk/objc/unittests/RTCCameraVideoCapturerTests.mm b/sdk/objc/unittests/RTCCameraVideoCapturerTests.mm
index 96bb53e..6235fb4 100644
--- a/sdk/objc/unittests/RTCCameraVideoCapturerTests.mm
+++ b/sdk/objc/unittests/RTCCameraVideoCapturerTests.mm
@@ -19,7 +19,7 @@
 
 #import "base/RTCVideoFrame.h"
 #import "components/capturer/RTCCameraVideoCapturer.h"
-#import "helpers/AVCaptureSession+RTCDevicePosition.h"
+#import "helpers/AVCaptureSession+DevicePosition.h"
 #import "helpers/RTCDispatcher.h"
 #import "helpers/scoped_cftyperef.h"
 
@@ -338,8 +338,8 @@
   CMSampleBufferRef sampleBuffer = createTestSampleBufferRef();
   [self setExif:sampleBuffer];
 
-  AVCaptureDevicePosition cameraPosition =
-      [AVCaptureSession rtc_devicePositionForSampleBuffer:sampleBuffer];
+  AVCaptureDevicePosition cameraPosition = [AVCaptureSession
+                                            devicePositionForSampleBuffer:sampleBuffer];
   EXPECT_EQ(cameraPosition, AVCaptureDevicePositionBack);
 #endif
 }
diff --git a/sdk/objc/unittests/RTCCertificateTest.mm b/sdk/objc/unittests/RTCCertificateTest.mm
index dace3df..bc13473 100644
--- a/sdk/objc/unittests/RTCCertificateTest.mm
+++ b/sdk/objc/unittests/RTCCertificateTest.mm
@@ -21,7 +21,7 @@
 #import "api/peerconnection/RTCMediaConstraints.h"
 #import "api/peerconnection/RTCPeerConnection.h"
 #import "api/peerconnection/RTCPeerConnectionFactory.h"
-#import "helpers/NSString+RTCStdString.h"
+#import "helpers/NSString+StdString.h"
 
 @interface RTCCertificateTest : XCTestCase
 @end
diff --git a/sdk/objc/unittests/RTCConfigurationTest.mm b/sdk/objc/unittests/RTCConfigurationTest.mm
index 1a10d56..18cc971 100644
--- a/sdk/objc/unittests/RTCConfigurationTest.mm
+++ b/sdk/objc/unittests/RTCConfigurationTest.mm
@@ -18,7 +18,7 @@
 #import "api/peerconnection/RTCConfiguration+Private.h"
 #import "api/peerconnection/RTCConfiguration.h"
 #import "api/peerconnection/RTCIceServer.h"
-#import "helpers/NSString+RTCStdString.h"
+#import "helpers/NSString+StdString.h"
 
 @interface RTCConfigurationTest : XCTestCase
 @end
diff --git a/sdk/objc/unittests/RTCDataChannelConfigurationTest.mm b/sdk/objc/unittests/RTCDataChannelConfigurationTest.mm
index e59876e..ccebd74 100644
--- a/sdk/objc/unittests/RTCDataChannelConfigurationTest.mm
+++ b/sdk/objc/unittests/RTCDataChannelConfigurationTest.mm
@@ -15,7 +15,7 @@
 
 #import "api/peerconnection/RTCDataChannelConfiguration+Private.h"
 #import "api/peerconnection/RTCDataChannelConfiguration.h"
-#import "helpers/NSString+RTCStdString.h"
+#import "helpers/NSString+StdString.h"
 
 @interface RTCDataChannelConfigurationTest : XCTestCase
 @end
diff --git a/sdk/objc/unittests/RTCIceCandidateTest.mm b/sdk/objc/unittests/RTCIceCandidateTest.mm
index 2b6eb20..5764119 100644
--- a/sdk/objc/unittests/RTCIceCandidateTest.mm
+++ b/sdk/objc/unittests/RTCIceCandidateTest.mm
@@ -17,7 +17,7 @@
 
 #import "api/peerconnection/RTCIceCandidate+Private.h"
 #import "api/peerconnection/RTCIceCandidate.h"
-#import "helpers/NSString+RTCStdString.h"
+#import "helpers/NSString+StdString.h"
 
 @interface RTCIceCandidateTest : XCTestCase
 @end
diff --git a/sdk/objc/unittests/RTCIceServerTest.mm b/sdk/objc/unittests/RTCIceServerTest.mm
index 1b3a12c..772653c 100644
--- a/sdk/objc/unittests/RTCIceServerTest.mm
+++ b/sdk/objc/unittests/RTCIceServerTest.mm
@@ -17,7 +17,7 @@
 
 #import "api/peerconnection/RTCIceServer+Private.h"
 #import "api/peerconnection/RTCIceServer.h"
-#import "helpers/NSString+RTCStdString.h"
+#import "helpers/NSString+StdString.h"
 
 @interface RTCIceServerTest : XCTestCase
 @end
@@ -125,10 +125,10 @@
       [[RTC_OBJC_TYPE(RTCIceServer) alloc] initWithNativeServer:nativeServer];
   EXPECT_EQ(1u, iceServer.urlStrings.count);
   EXPECT_EQ("stun:stun.example.net",
-            [NSString rtc_stdStringForString:iceServer.urlStrings.firstObject]);
-  EXPECT_EQ("username", [NSString rtc_stdStringForString:iceServer.username]);
-  EXPECT_EQ("password", [NSString rtc_stdStringForString:iceServer.credential]);
-  EXPECT_EQ("hostname", [NSString rtc_stdStringForString:iceServer.hostname]);
+      [NSString stdStringForString:iceServer.urlStrings.firstObject]);
+  EXPECT_EQ("username", [NSString stdStringForString:iceServer.username]);
+  EXPECT_EQ("password", [NSString stdStringForString:iceServer.credential]);
+  EXPECT_EQ("hostname", [NSString stdStringForString:iceServer.hostname]);
   EXPECT_EQ(2u, iceServer.tlsAlpnProtocols.count);
   EXPECT_EQ(2u, iceServer.tlsEllipticCurves.count);
 }
diff --git a/sdk/objc/unittests/RTCMediaConstraintsTest.mm b/sdk/objc/unittests/RTCMediaConstraintsTest.mm
index 57a3fb8..6ed7859 100644
--- a/sdk/objc/unittests/RTCMediaConstraintsTest.mm
+++ b/sdk/objc/unittests/RTCMediaConstraintsTest.mm
@@ -17,7 +17,7 @@
 
 #import "api/peerconnection/RTCMediaConstraints+Private.h"
 #import "api/peerconnection/RTCMediaConstraints.h"
-#import "helpers/NSString+RTCStdString.h"
+#import "helpers/NSString+StdString.h"
 
 @interface RTCMediaConstraintsTests : XCTestCase
 @end
diff --git a/sdk/objc/unittests/RTCPeerConnectionTest.mm b/sdk/objc/unittests/RTCPeerConnectionTest.mm
index 45c9ce5..9ca8403 100644
--- a/sdk/objc/unittests/RTCPeerConnectionTest.mm
+++ b/sdk/objc/unittests/RTCPeerConnectionTest.mm
@@ -26,7 +26,7 @@
 #import "api/peerconnection/RTCPeerConnectionFactory+Native.h"
 #import "api/peerconnection/RTCPeerConnectionFactory.h"
 #import "api/peerconnection/RTCSessionDescription.h"
-#import "helpers/NSString+RTCStdString.h"
+#import "helpers/NSString+StdString.h"
 
 @interface RTCPeerConnectionTests : XCTestCase
 @end
diff --git a/sdk/objc/unittests/RTCSessionDescriptionTest.mm b/sdk/objc/unittests/RTCSessionDescriptionTest.mm
index fcacc26..70c82f7 100644
--- a/sdk/objc/unittests/RTCSessionDescriptionTest.mm
+++ b/sdk/objc/unittests/RTCSessionDescriptionTest.mm
@@ -15,7 +15,7 @@
 
 #import "api/peerconnection/RTCSessionDescription+Private.h"
 #import "api/peerconnection/RTCSessionDescription.h"
-#import "helpers/NSString+RTCStdString.h"
+#import "helpers/NSString+StdString.h"
 
 @interface RTCSessionDescriptionTests : XCTestCase
 @end
diff --git a/sdk/objc/unittests/RTCTracingTest.mm b/sdk/objc/unittests/RTCTracingTest.mm
index 133a54d..ff93047 100644
--- a/sdk/objc/unittests/RTCTracingTest.mm
+++ b/sdk/objc/unittests/RTCTracingTest.mm
@@ -16,7 +16,7 @@
 #include "rtc_base/gunit.h"
 
 #import "api/peerconnection/RTCTracing.h"
-#import "helpers/NSString+RTCStdString.h"
+#import "helpers/NSString+StdString.h"
 
 @interface RTCTracingTests : XCTestCase
 @end