Partial revert of Enable -Winconsistent-missing-override flag. (patchset #5 id:80001 of https://codereview.webrtc.org/1921653002/ )

Reason for revert:
This CL breaks the google3 import (but not the import bot).
This partial revert only reverts the build files. A full revert no longer cleanly applies to ToT, so this was done instead.

Original issue's description:
> Enable -Winconsistent-missing-override flag.
>
> The problem with gmock is worked around by commenting out any other override declarations in classes using gmock.
>
> NOPRESUBMIT=True
> BUG=webrtc:3970
>
> Committed: https://crrev.com/ef8b61e11062295365f11b9942f18a08a8b3ec60
> Cr-Commit-Position: refs/heads/master@{#12563}

TBR=mflodman@webrtc.org,kjellander@webrtc.org,nisse@webrtc.org
BUG=webrtc:3970

Review-Url: https://codereview.webrtc.org/1944273002
Cr-Commit-Position: refs/heads/master@{#12624}

webrtc/BUILD.gn

@@ -117,7 +117,6 @@ config("common_config") {
       cflags += [
         "-Wimplicit-fallthrough",
         "-Wthread-safety",
-        "-Winconsistent-missing-override",
       ]
     }
   }

webrtc/build/common.gypi

@@ -313,7 +313,6 @@
             'cflags': [
               '-Wimplicit-fallthrough',
               '-Wthread-safety',
-              '-Winconsistent-missing-override',
             ],
           }],
         ],