Remove duplicated or non effective build flags.

-Winconsistent-missing-override is part of -Wall so there is no need
to explicitly set it.

-Wthread-safety and -Wimplicit-fallthrough are set by default by
Chromium's toolchain and there is no need to duplicated it in WebRTC
GN files:

gn desc out/Debug/ //rtc_base cflags | grep "Wthread-safety"
-Wthread-safety
-Wthread-safety

gn desc out/Debug/ //rtc_base cflags | grep "implicit"
-Wimplicit-fallthrough
-Wimplicit-fallthrough

Bug: None
Change-Id: Ie5104f7c6d508c7b45788420bf111a17b8b10939
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/226868
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#34549}

BUILD.gn

@@ -344,9 +344,6 @@ config("common_config") {
     if (is_clang) {
       cflags += [
         "-Wc++11-narrowing",
-        "-Wimplicit-fallthrough",
-        "-Wthread-safety",
-        "-Winconsistent-missing-override",
         "-Wundef",
       ]