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/ns/include/noise_suppression.h

@@ -41,12 +41,8 @@ int WebRtcNs_Create(NsHandle** NS_inst);
  *
  * Input:
  *      - NS_inst       : Pointer to NS instance that should be freed
- *
- * Return value         :  0 - Ok
- *                        -1 - Error
  */
-int WebRtcNs_Free(NsHandle* NS_inst);
-
+void WebRtcNs_Free(NsHandle* NS_inst);
 
 /*
  * This function initializes a NS instance and has to be called before any other