Remove all old suspension logic.

I'm also removing media_optimization_unittest.cc, since it only tested the
suspension logic and nothing else.

R=pbos@webrtc.org

Review URL: https://codereview.webrtc.org/2119503002 .

Cr-Commit-Position: refs/heads/master@{#13355}

webrtc/modules/video_coding/video_coding_impl.h

@@ -89,9 +89,6 @@ class VideoSender : public Module {
   int32_t IntraFrameRequest(size_t stream_index);
   int32_t EnableFrameDropper(bool enable);
 
-  void SuspendBelowMinBitrate();
-  bool VideoSuspended() const;
-
   int64_t TimeUntilNextProcess() override;
   void Process() override;