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/system_delay_unittest.cc

@@ -73,7 +73,7 @@ void SystemDelayTest::SetUp() {
 
 void SystemDelayTest::TearDown() {
   // Free AEC
-  ASSERT_EQ(0, WebRtcAec_Free(handle_));
+  WebRtcAec_Free(handle_);
   handle_ = NULL;
 }