Remove ignored return code from modules.

ModuleProcessImpl doesn't act on return codes and having them around is
confusing (it's unclear what an error return code here would do even).

BUG=
R=tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1703833002 .

Cr-Commit-Position: refs/heads/master@{#11747}

webrtc/modules/bitrate_controller/bitrate_controller_impl.h

@@ -45,7 +45,7 @@ class BitrateControllerImpl : public BitrateController {
   void SetEventLog(RtcEventLog* event_log) override;
 
   int64_t TimeUntilNextProcess() override;
-  int32_t Process() override;
+  void Process() override;
 
  private:
   class RtcpBandwidthObserverImpl;