Set RtpReceiverAudio::telephone_event_forward_to_decoder_ true on construction.

All users call SetTelephoneEventForwardToDecoder(true). Setting the
flag to true on construction, enables deletion of those calls,
followed by deletion of the flag itself.

The unused getter method TelephoneEventForwardToDecoder() is deleted
right away.

Bug: webrtc:7135
Change-Id: I8c52c957b3f074be7ffc425b3588402d1e42b844
Reviewed-on: https://webrtc-review.googlesource.com/90402
Reviewed-by: Oskar Sundbom <ossu@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24141}

modules/rtp_rtcp/include/rtp_receiver.h

@@ -29,9 +29,6 @@ class TelephoneEventHandler {
   // Forward DTMFs to decoder for playout.
   virtual void SetTelephoneEventForwardToDecoder(bool forward_to_decoder) = 0;
 
-  // Is forwarding of outband telephone events turned on/off?
-  virtual bool TelephoneEventForwardToDecoder() const = 0;
-
   // Is TelephoneEvent configured with payload type payload_type
   virtual bool TelephoneEventPayloadType(const int8_t payload_type) const = 0;
 };