remove mslabel and mslabel ssrc-specific attributes

Removes support for parsing and serializing
  a=ssrc:1 mslabel:stream
  a=ssrc:1 label:track
which have been superceeded by
  a=ssrc:1 msid:stream track
a long time ago.

Bug: webrtc:7110
Change-Id: I3aca47728098b6e7e049b82ed34c59426d411c41
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168244
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30801}
diff --git a/sdk/objc/unittests/RTCSessionDescriptionTest.mm b/sdk/objc/unittests/RTCSessionDescriptionTest.mm
index 0807eed..6acfad9 100644
--- a/sdk/objc/unittests/RTCSessionDescriptionTest.mm
+++ b/sdk/objc/unittests/RTCSessionDescriptionTest.mm
@@ -90,8 +90,6 @@
           "a=maxptime:60\r\n"
           "a=ssrc:1504474588 cname:V+FdIC5AJpxLhdYQ\r\n"
           "a=ssrc:1504474588 msid:ARDAMS ARDAMSa0\r\n"
-          "a=ssrc:1504474588 mslabel:ARDAMS\r\n"
-          "a=ssrc:1504474588 label:ARDAMSa0\r\n"
           "m=video 9 UDP/TLS/RTP/SAVPF 100 116 117 96\r\n"
           "c=IN IP4 0.0.0.0\r\n"
           "a=rtcp:9 IN IP4 0.0.0.0\r\n"
@@ -120,12 +118,8 @@
           "a=ssrc-group:FID 498297514 1644357692\r\n"
           "a=ssrc:498297514 cname:V+FdIC5AJpxLhdYQ\r\n"
           "a=ssrc:498297514 msid:ARDAMS ARDAMSv0\r\n"
-          "a=ssrc:498297514 mslabel:ARDAMS\r\n"
-          "a=ssrc:498297514 label:ARDAMSv0\r\n"
           "a=ssrc:1644357692 cname:V+FdIC5AJpxLhdYQ\r\n"
-          "a=ssrc:1644357692 msid:ARDAMS ARDAMSv0\r\n"
-          "a=ssrc:1644357692 mslabel:ARDAMS\r\n"
-          "a=ssrc:1644357692 label:ARDAMSv0\r\n";
+          "a=ssrc:1644357692 msid:ARDAMS ARDAMSv0\r\n";
 }
 
 @end