Remove deprecated field_trial_default and metrics_default.

This CL removes some deprecated build targets (and their headers)
from system_wrappers:
- field_trial_api
- field_trial_default
- metrics_api
- metrics_default

It also refreshes all the dependencies on field_trial.h and metrics.h.

A nice side effect is that it is finally possible to remove 'nogncheck'
from the following files (when it was used with field_trial_default
and metrics_default):
- sdk/objc/api/peerconnection/RTCMetricsSampleInfo+Private.h
- sdk/android/src/jni/pc/peerconnectionfactory.cc
- sdk/objc/api/peerconnection/RTCFieldTrials.mm

Bug: webrtc:9631
Change-Id: Ib621f41ef8ad0aba4fe1c1d7e749c044afc956c3
No-Try: True
Reviewed-on: https://webrtc-review.googlesource.com/100524
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24878}
diff --git a/p2p/BUILD.gn b/p2p/BUILD.gn
index 0f08990..2d351bb 100644
--- a/p2p/BUILD.gn
+++ b/p2p/BUILD.gn
@@ -94,8 +94,8 @@
     "../rtc_base:weak_ptr",
     "../rtc_base/third_party/base64",
     "../rtc_base/third_party/sigslot",
-    "../system_wrappers:field_trial_api",
-    "../system_wrappers:metrics_api",
+    "../system_wrappers:field_trial",
+    "../system_wrappers:metrics",
     "//third_party/abseil-cpp/absl/memory",
     "//third_party/abseil-cpp/absl/types:optional",
   ]
@@ -182,7 +182,7 @@
       "../rtc_base:rtc_base_approved",
       "../rtc_base:rtc_base_tests_utils",
       "../rtc_base:stringutils",
-      "../system_wrappers:metrics_default",
+      "../system_wrappers:metrics",
       "../test:test_support",
       "//testing/gtest",
       "//third_party/abseil-cpp/absl/memory",