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}
diff --git a/webrtc/voice_engine/monitor_module.h b/webrtc/voice_engine/monitor_module.h
index 5539915..f026f72 100644
--- a/webrtc/voice_engine/monitor_module.h
+++ b/webrtc/voice_engine/monitor_module.h
@@ -42,7 +42,7 @@
 public:	// module
  int64_t TimeUntilNextProcess() override;
 
- int32_t Process() override;
+ void Process() override;
 
 private:
     rtc::CriticalSection _callbackCritSect;