Remove unused/deprecated DTMF methods

PeerConnectionInterface::CreateDtmfSender
DtmfSenderInterface::track

Bug: webrtc:9426
Change-Id: I7d151d8e0bdd60750ed60466083245631d540a91
Reviewed-on: https://webrtc-review.googlesource.com/84244
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23690}

api/dtmfsenderinterface.h

@@ -71,12 +71,6 @@ class DtmfSenderInterface : public rtc::RefCountInterface {
                           int duration,
                           int inter_tone_gap) = 0;
 
-  // Returns the track given as argument to the constructor. Only exists for
-  // backwards compatibilty; now that DtmfSenders are tied to RtpSenders, it's
-  // no longer relevant.
-  // TODO(bugs.webrtc.org/9426): Remove this method.
-  virtual const AudioTrackInterface* track() const { return nullptr; }
-
   // Returns the tones remaining to be played out.
   virtual std::string tones() const = 0;