Remove the VoiceEngineObserver callback interface.

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("../webrtc.gni")
 
 rtc_static_library("voice_engine") {
   sources = [
     "channel.cc",
     "channel.h",
     "channel_manager.cc",
     "channel_manager.h",
     "channel_proxy.cc",
     "channel_proxy.h",
     "include/voe_base.h",
     "include/voe_errors.h",
-    "monitor_module.h",
     "shared_data.cc",
     "shared_data.h",
     "statistics.cc",
     "statistics.h",
     "transmit_mixer.cc",
     "transmit_mixer.h",
     "transport_feedback_packet_loss_tracker.cc",
     "transport_feedback_packet_loss_tracker.h",
     "utility.cc",
     "utility.h",
@@ skipping 115 matching lines @@
         "/wd4373",  # Virtual function override.
       ]
     }
 
     if (!build_with_chromium && is_clang) {
       # Suppress warnings from the Chromium Clang plugin (bugs.webrtc.org/163).
       suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]
     }
   }
 }