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/congestion_controller/include/congestion_controller.h

@@ -65,7 +65,7 @@ class CongestionController : public CallStatsObserver, public Module {
 
   // Implements Module.
   int64_t TimeUntilNextProcess() override;
-  int32_t Process() override;
+  void Process() override;
 
  private:
   Clock* const clock_;