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.h

@@ -23,7 +23,7 @@ enum {
 // Unless otherwise specified, functions return 0 on success and -1 on error
 int WebRtcAec_CreateResampler(void** resampInst);
 int WebRtcAec_InitResampler(void* resampInst, int deviceSampleRateHz);
-int WebRtcAec_FreeResampler(void* resampInst);
+void WebRtcAec_FreeResampler(void* resampInst);
 
 // Estimates skew from raw measurement.
 int WebRtcAec_GetSkew(void* resampInst, int rawSkew, float* skewEst);