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/codecs/i420/main/interface/i420.h

@@ -102,11 +102,6 @@ class I420Decoder : public VideoDecoder {
   int InitDecode(const VideoCodec* codecSettings,
                  int /*numberOfCores*/) override;
 
-  int SetCodecConfigParameters(const uint8_t* /*buffer*/,
-                               int /*size*/) override {
-    return WEBRTC_VIDEO_CODEC_OK;
-  }
-
 // Decode encoded image (as a part of a video stream). The decoded image
 // will be returned to the user through the decode complete callback.
 //