Refactor audio_processing: Free functions return void

There is no point in returning an error when Free() fails. In fact it can only happen if we have a null pointer as object. There is further no place where the return value is used.

Affected components are
- aec
- aecm
- agc
- ns

BUG=441
R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/50579004

Cr-Commit-Position: refs/heads/master@{#8966}

webrtc/modules/audio_processing/aec/aec_resampler.c

@@ -63,11 +63,9 @@ int WebRtcAec_InitResampler(void* resampInst, int deviceSampleRateHz) {
   return 0;
 }
 
-int WebRtcAec_FreeResampler(void* resampInst) {
+void WebRtcAec_FreeResampler(void* resampInst) {
   AecResampler* obj = (AecResampler*)resampInst;
   free(obj);
-
-  return 0;
 }
 
 void WebRtcAec_ResampleLinear(void* resampInst,