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/include/video_coding.h

@@ -394,12 +394,6 @@ class VideoCodingModule : public Module {
   //                     < 0,               on error.
   virtual int32_t Delay() const = 0;
 
-  // Returns the number of packets discarded by the jitter buffer due to being
-  // too late. This can include duplicated packets which arrived after the
-  // frame was sent to the decoder. Therefore packets which were prematurely
-  // NACKed will be counted.
-  virtual uint32_t DiscardedPackets() const = 0;
-
   // Robustness APIs
 
   // Set the receiver robustness mode. The mode decides how the receiver