Stop setting -Wextra (the toolchain already does that).

The comment was stale and setting -Wextra actually turns on diagnostics
that are turned off by Chromium.

For example:
"-Wextra -Wno-deprecated-copy -Wextra" will turn on -Wdeprecated-copy
because starting from https://reviews.llvm.org/D70342
-Wdeprecated-copy is part of -Wextra.

Bug: webrtc:11180
Change-Id: Ia5d1e22bfe42d67cd892ae07620e7fd2daf9a7a4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/161332
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30019}

BUILD.gn

@@ -271,6 +271,8 @@ config("common_config") {
     defines += [ "RTC_DISABLE_TRACE_EVENTS" ]
   }
 
+  cflags = []
+
   if (build_with_chromium) {
     defines += [
       # NOTICE: Since common_inherited_config is used in public_configs for our
@@ -282,14 +284,6 @@ config("common_config") {
     ]
   } else {
     if (is_posix || is_fuchsia) {
-      # Enable more warnings: -Wextra is currently disabled in Chromium.
-      cflags = [
-        "-Wextra",
-
-        # Repeat some flags that get overridden by -Wextra.
-        "-Wno-unused-parameter",
-        "-Wno-missing-field-initializers",
-      ]
       cflags_c += [
         # TODO(bugs.webrtc.org/9029): enable commented compiler flags.
         # Some of these flags should also be added to cflags_objc.