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/modules/remote_bitrate_estimator/BUILD.gn b/modules/remote_bitrate_estimator/BUILD.gn
index 63564a6..f5fccbd 100644
--- a/modules/remote_bitrate_estimator/BUILD.gn
+++ b/modules/remote_bitrate_estimator/BUILD.gn
@@ -48,8 +48,8 @@
     "../../rtc_base:rtc_base_approved",
     "../../rtc_base:safe_minmax",
     "../../system_wrappers",
-    "../../system_wrappers:field_trial_api",
-    "../../system_wrappers:metrics_api",
+    "../../system_wrappers:field_trial",
+    "../../system_wrappers:metrics",
     "//third_party/abseil-cpp/absl/types:optional",
   ]
 }
@@ -134,7 +134,7 @@
       "../../rtc_base/system:fallthrough",
       "../../rtc_base/system:unused",
       "../../system_wrappers",
-      "../../system_wrappers:field_trial_api",
+      "../../system_wrappers:field_trial",
       "../../test:fileutils",
       "../../test:perf_test",
       "../../test:test_support",