Remove redudant encoder rate calls.

Moves EncoderParameters update checks into GenericEncoder before calling
SetRates/SetChannelParameters as applicable.

Also removes CodecConfigParameters as a bonus.

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

Review URL: https://codereview.webrtc.org/1426953003 .

Cr-Commit-Position: refs/heads/master@{#10452}

webrtc/modules/video_coding/codecs/i420/include/i420.h

@@ -73,10 +73,6 @@ class I420Encoder : public VideoEncoder {
     return WEBRTC_VIDEO_CODEC_OK;
   }
 
-  int CodecConfigParameters(uint8_t* /*buffer*/, int /*size*/) override {
-    return WEBRTC_VIDEO_CODEC_OK;
-  }
-
   void OnDroppedFrame() override {}
 
  private: