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_coding_impl.h

@@ -179,7 +179,6 @@ class VideoReceiver : public Module {
   int32_t SetMinimumPlayoutDelay(uint32_t minPlayoutDelayMs);
   int32_t SetRenderDelay(uint32_t timeMS);
   int32_t Delay() const;
-  uint32_t DiscardedPackets() const;
 
   int SetReceiverRobustnessMode(ReceiverRobustness robustnessMode,
                                 VCMDecodeErrorMode errorMode);