Remove unimplemented function declarations VCMGenericDecoder::External and VCMTimestampMap::Reset.

Bug: none
Change-Id: I6dfaad8f6b770c0d141879b3686e229a01ac5975
Reviewed-on: https://webrtc-review.googlesource.com/97120
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Commit-Queue: Åsa Persson <asapersson@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24599}

modules/video_coding/timestamp_map.h

@@ -22,9 +22,6 @@ class VCMTimestampMap {
   explicit VCMTimestampMap(size_t capacity);
   ~VCMTimestampMap();
 
-  // Empty the map.
-  void Reset();
-
   void Add(uint32_t timestamp, VCMFrameInformation* data);
   VCMFrameInformation* Pop(uint32_t timestamp);