Delete unused method VideoCodingModule::DiscardedPackets().

This method isn't called and the value it represents, is made available
via the stats APIs.

BUG=none

Review-Url: https://codereview.webrtc.org/2760613002
Cr-Commit-Position: refs/heads/master@{#17287}

webrtc/modules/video_coding/video_receiver.cc

@@ -411,10 +411,6 @@ int32_t VideoReceiver::Delay() const {
   return _timing->TargetVideoDelay();
 }
 
-uint32_t VideoReceiver::DiscardedPackets() const {
-  return _receiver.DiscardedPackets();
-}
-
 int VideoReceiver::SetReceiverRobustnessMode(
     ReceiverRobustness robustnessMode,
     VCMDecodeErrorMode decode_error_mode) {