Remove VoEVideoSync interface.

The removed tests are covered by cases in call_perf_tests.

BUG=webrtc:4690

Review-Url: https://codereview.webrtc.org/2672583002
Cr-Commit-Position: refs/heads/master@{#16621}

webrtc/modules/audio_coding/include/audio_coding_module.h

@@ -637,7 +637,8 @@ class AudioCodingModule {
   //
   virtual int SetMaximumPlayoutDelay(int time_ms) = 0;
 
-  //
+  // TODO(kwiberg): Consider if this is needed anymore, now that voe::Channel
+  //                doesn't use it.
   // The shortest latency, in milliseconds, required by jitter buffer. This
   // is computed based on inter-arrival times and playout mode of NetEq. The
   // actual delay is the maximum of least-required-delay and the minimum-delay