MediaStream API: Deleting RTCDTMFSenderHandler
Removing the unnecessary bridging class RTCDTMFSenderHandler as part of
the effort to get rid of Source/core/platform/mediastream/. This class
doesn't reside in Source/core/platform/mediastream/ but getting rid of
it now makes it easier to delete RTCPeerConnectionHandler later.

Review URL: https://codereview.chromium.org/143873011

git-svn-id: svn://svn.chromium.org/blink/trunk@165593 bbb929c8-8fbe-4397-9dbb-9b2b20218538