Delete unused enum VideoCodingModule::SenderNackMode.

It appesrs unused for a long time; an alias was deleted in
https://webrtc-review.googlesource.com/c/124488, but it was already
unused.

Bug: None
Change-Id: Idae6a72949968e22c784d512f9617240ef1169b4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/128569
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27187}

modules/video_coding/include/video_coding.h

@@ -29,8 +29,6 @@ struct CodecSpecificInfo;
 
 class VideoCodingModule : public Module {
  public:
-  enum SenderNackMode { kNackNone, kNackAll, kNackSelective };
-
   // DEPRECATED.
   static VideoCodingModule* Create(Clock* clock);