VoE Channel: Don't register codecs when stopping receiver

VoiceEngine's Channel::StopReceiving() would call
RegisterReceiveCodecsToRTPModule(), which caused some errors
with RED and ULP-FEC. In particular, an error message would be
printed when hanging up a call in voe_cmd_test application.

BUG=3085
R=henrika@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/10779004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@5811 4adac7df-926f-26a2-2b94-8c16560cd09d