Deleted declaration of VideoCaptureInput::DeliverI420Frame

It appears unimplemented and unused.

BUG=

Review URL: https://codereview.webrtc.org/1539513002

Cr-Commit-Position: refs/heads/master@{#11071}

webrtc/video/video_capture_input.h

@@ -65,8 +65,6 @@ class VideoCaptureInput : public webrtc::VideoCaptureInput {
   static bool EncoderThreadFunction(void* obj);
   bool EncoderProcess();
 
-  void DeliverI420Frame(VideoFrame* video_frame);
-
   rtc::scoped_ptr<CriticalSectionWrapper> capture_cs_;
   ProcessThread* const module_process_thread_;