Remove unused method OutputMixer::PlayDtmfTone() and infrastructure.

webrtc/voice_engine/BUILD.gn

 # Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("../build/webrtc.gni")
 
 source_set("voice_engine") {
   sources = [
     "channel.cc",
     "channel.h",
     "channel_manager.cc",
     "channel_manager.h",
     "channel_proxy.cc",
     "channel_proxy.h",
-    "dtmf_inband.cc",
-    "dtmf_inband.h",
     "include/voe_audio_processing.h",
     "include/voe_base.h",
     "include/voe_codec.h",
     "include/voe_errors.h",
     "include/voe_external_media.h",
     "include/voe_file.h",
     "include/voe_hardware.h",
     "include/voe_neteq_stats.h",
     "include/voe_network.h",
     "include/voe_rtp_rtcp.h",
@@ skipping 70 matching lines @@
     "../modules/audio_device",
     "../modules/audio_processing",
     "../modules/bitrate_controller",
     "../modules/media_file",
     "../modules/pacing",
     "../modules/rtp_rtcp",
     "../modules/utility",
     "../system_wrappers",
   ]
 }