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}

sdk/android/src/jni/androidmetrics.cc

@@ -15,7 +15,6 @@
 #include "sdk/android/native_api/jni/java_types.h"
 #include "sdk/android/src/jni/jni_helpers.h"
 #include "system_wrappers/include/metrics.h"
-#include "system_wrappers/include/metrics_default.h"
 
 // Enables collection of native histograms and creating them.
 namespace webrtc {

sdk/android/src/jni/pc/peerconnectionfactory.cc

@@ -37,10 +37,6 @@
 #include "sdk/android/src/jni/pc/sslcertificateverifierwrapper.h"
 #include "sdk/android/src/jni/pc/video.h"
 #include "system_wrappers/include/field_trial.h"
-// Adding 'nogncheck' to disable the gn include headers check.
-// We don't want to depend on 'system_wrappers:field_trial_default' because
-// clients should be able to provide their own implementation.
-#include "system_wrappers/include/field_trial_default.h"  // nogncheck
 
 namespace webrtc {
 namespace jni {