Remove unused VideoDecoder methods.

Removing VideoDecoder::Copy() and
VideoDecoder::SetCodecConfigParameters().

Also adding override to VP8DecoderImpl.

BUG=
R=mflodman@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/55409004

Cr-Commit-Position: refs/heads/master@{#9244}

webrtc/modules/video_coding/main/source/generic_decoder.cc

@@ -186,11 +186,6 @@ int32_t VCMGenericDecoder::Reset()
     return _decoder.Reset();
 }
 
-int32_t VCMGenericDecoder::SetCodecConfigParameters(const uint8_t* buffer, int32_t size)
-{
-    return _decoder.SetCodecConfigParameters(buffer, size);
-}
-
 int32_t VCMGenericDecoder::RegisterDecodeCompleteCallback(VCMDecodedFrameCallback* callback)
 {
     _callback = callback;