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/modules/media_file/media_file_impl.cc b/webrtc/modules/media_file/media_file_impl.cc
index abc7b9d..76bcca7 100644
--- a/webrtc/modules/media_file/media_file_impl.cc
+++ b/webrtc/modules/media_file/media_file_impl.cc
@@ -95,11 +95,10 @@
     return -1;
 }
 
-int32_t MediaFileImpl::Process()
+void MediaFileImpl::Process()
 {
     WEBRTC_TRACE(kTraceWarning, kTraceFile, _id,
                  "Process: This method is not used by MediaFile class.");
-    return -1;
 }
 
 int32_t MediaFileImpl::PlayoutAudioData(int8_t* buffer,