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/rtp_rtcp/include/receive_statistics.h

@@ -90,7 +90,7 @@ class NullReceiveStatistics : public ReceiveStatistics {
   StatisticianMap GetActiveStatisticians() const override;
   StreamStatistician* GetStatistician(uint32_t ssrc) const override;
   int64_t TimeUntilNextProcess() override;
-  int32_t Process() override;
+  void Process() override;
   void SetMaxReorderingThreshold(int max_reordering_threshold) override;
   void RegisterRtcpStatisticsCallback(
       RtcpStatisticsCallback* callback) override;