Removing clang-cl warning suppression flags in rtc_base.

Bug: webrtc:9251
Change-Id: I06112d1d683a65b1555b5201deb236928dd8d196
Reviewed-on: https://webrtc-review.googlesource.com/77002
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23259}

rtc_base/BUILD.gn

@@ -647,16 +647,6 @@ rtc_static_library("rtc_numerics") {
   ]
 }
 
-config("rtc_base_warnings_config") {
-  if (is_win && is_clang) {
-    cflags = [
-      # Disable warnings failing when compiling with Clang on Windows.
-      # https://bugs.chromium.org/p/webrtc/issues/detail?id=5366
-      "-Wno-sign-compare",
-    ]
-  }
-}
-
 rtc_source_set("rtc_json") {
   defines = []
   sources = [
@@ -855,7 +845,6 @@ rtc_static_library("rtc_base_generic") {
     include_dirs = [ "../../boringssl/src/include" ]
     public_configs += [ ":rtc_base_chromium_config" ]
   } else {
-    configs += [ ":rtc_base_warnings_config" ]
     sources += [
       "callback.h",
       "logsinks.cc",