Remove double declaration of cricket::kH264CodecName.

This symbol is declared in two places and clang-cl complains if they
disagree on the visibility (see [1]), since this declaration is
redundant this CL just removes it.

[1] - https://ci.chromium.org/p/chromium/builders/try/win_chromium_compile_dbg_ng/432927

Bug: webrtc:9419
Change-Id: If2f04beff8fd757ec2019c3588c82b6971f012c3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/159029
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29729}

media/engine/constants.h

@@ -17,8 +17,6 @@ extern const int kVideoMtu;
 extern const int kVideoRtpSendBufferSize;
 extern const int kVideoRtpRecvBufferSize;
 
-extern const char kH264CodecName[];
-
 }  // namespace cricket
 
 #endif  // MEDIA_ENGINE_CONSTANTS_H_