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.cc b/webrtc/voice_engine/monitor_module.cc
index 2d8f4d4..fb3c2b4 100644
--- a/webrtc/voice_engine/monitor_module.cc
+++ b/webrtc/voice_engine/monitor_module.cc
@@ -57,7 +57,7 @@
     return kAverageProcessUpdateTimeMs - (now - _lastProcessTime);
 }
 
-int32_t
+void
 MonitorModule::Process()
 {
     _lastProcessTime = TickTime::MillisecondTimestamp();
@@ -66,7 +66,6 @@
     {
         _observerPtr->OnPeriodicProcess();
     }
-    return 0;
 }
 
 }  // namespace voe