Removes parts of the webrtc::VoEDtmf sub API as part of a clean-up operation where the goal is to remove unused APIs.

BUG=3206
R=wu@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/12299005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6146 4adac7df-926f-26a2-2b94-8c16560cd09d