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/BUILD.gn b/BUILD.gn
index 3f06fa9..f3aae92 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -401,11 +401,7 @@
     ]
 
     if (build_with_mozilla) {
-      deps += [
-        "api/video:video_frame",
-        "system_wrappers:field_trial_default",
-        "system_wrappers:metrics_default",
-      ]
+      deps += [ "api/video:video_frame" ]
     } else {
       deps += [
         "api",
@@ -475,7 +471,6 @@
       "rtc_base:sigslot_unittest",
       "rtc_base:weak_ptr_unittests",
       "rtc_base/experiments:experiments_unittests",
-      "system_wrappers:metrics_default",
       "system_wrappers:runtime_enabled_features_default",
     ]