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.cc

@@ -152,12 +152,6 @@ class VideoCodingModuleImpl : public VideoCodingModule {
     return sender_.EnableFrameDropper(enable);
   }
 
-  void SuspendBelowMinBitrate() override {
-    return sender_.SuspendBelowMinBitrate();
-  }
-
-  bool VideoSuspended() const override { return sender_.VideoSuspended(); }
-
   int32_t RegisterReceiveCodec(const VideoCodec* receiveCodec,
                                int32_t numberOfCores,
                                bool requireKeyFrame) override {