Convert CNG into C++ and remove it from AudioDecoder

webrtc/modules/audio_coding/codecs/audio_decoder.cc

 /*
  *  Copyright (c) 2012 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.
  */
 
@@ skipping 64 matching lines @@
 int AudioDecoder::PacketDurationRedundant(const uint8_t* encoded,
                                           size_t encoded_len) const {
   return kNotImplemented;
 }
 
 bool AudioDecoder::PacketHasFec(const uint8_t* encoded,
                                 size_t encoded_len) const {
   return false;
 }
 
-CNG_dec_inst* AudioDecoder::CngDecoderInstance() {
-  FATAL() << "Not a CNG decoder";
-  return NULL;
-}
-
 AudioDecoder::SpeechType AudioDecoder::ConvertSpeechType(int16_t type) {
   switch (type) {
     case 0:  // TODO(hlundin): Both iSAC and Opus return 0 for speech.
     case 1:
       return kSpeech;
     case 2:
       return kComfortNoise;
     default:
       assert(false);
       return kSpeech;
   }
 }
 
 }  // namespace webrtc