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/rtc_tools/BUILD.gn b/rtc_tools/BUILD.gn
index 437dca4..990d964 100644
--- a/rtc_tools/BUILD.gn
+++ b/rtc_tools/BUILD.gn
@@ -305,7 +305,7 @@
         "../logging:rtc_event_log_parser",
         "../rtc_base:protobuf_utils",
         "../rtc_base:rtc_base_approved",
-        "../system_wrappers:field_trial_default",
+        "../system_wrappers:field_trial",
         "../test:field_trial",
         "../test:fileutils",
         "../test:test_support",
@@ -325,7 +325,6 @@
       "../modules/audio_processing/vad",
       "../rtc_base:rtc_base_approved",
       "../rtc_base:safe_minmax",
-      "../system_wrappers:metrics_default",
       "../test:test_support",
       "//testing/gtest",
     ]