Small refactoring of AudioProcessing use in channel.cc.

- Apply consistent naming.
- Use a scoped_ptr for rx_audioproc_.
- Remove now unnecessary AudioProcessing::Destroy().

R=bjornv@webrtc.org, xians@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/2184007

git-svn-id: http://webrtc.googlecode.com/svn/trunk@4784 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/audio_processing/audio_processing_impl.cc

@@ -47,10 +47,6 @@ AudioProcessing* AudioProcessing::Create(int id) {
   return apm;
 }
 
-void AudioProcessing::Destroy(AudioProcessing* apm) {
-  delete static_cast<AudioProcessingImpl*>(apm);
-}
-
 int32_t AudioProcessing::TimeUntilNextProcess() { return -1; }
 int32_t AudioProcessing::Process() { return -1; }