Remove ReceiveCodec() getters from VideoCodingModule.

The getters are not used and the implementation cannot be guaranteed
to return a correct value except when called synchronously from
the decoding thread while decoding.

The methods as is imply that the implementation needs to offer some
sort of synchronization, and that's not desirable.

BUG=webrtc:7328
R=stefan@webrtc.org

Review-Url: https://codereview.webrtc.org/2741853008 .
Cr-Commit-Position: refs/heads/master@{#17233}

webrtc/modules/video_coding/video_coding_impl.h

@@ -175,9 +175,6 @@ class VideoReceiver : public Module {
   // Called on the decoder thread when thread is exiting.
   void DecodingStopped();
 
-  int32_t ReceiveCodec(VideoCodec* currentReceiveCodec) const;
-  VideoCodecType ReceiveCodec() const;
-
   int32_t IncomingPacket(const uint8_t* incomingPayload,
                          size_t payloadLength,
                          const WebRtcRTPHeader& rtpInfo);