Convert CNG into C++ and remove it from AudioDecoder

webrtc/modules/audio_coding/codecs/audio_decoder.cc

Index: webrtc/modules/audio_coding/codecs/audio_decoder.cc
diff --git a/webrtc/modules/audio_coding/codecs/audio_decoder.cc b/webrtc/modules/audio_coding/codecs/audio_decoder.cc
index d2984b97b0918d4081c5cf7dc5f403d532d799cc..442ddc1e4b86b081653da168b363bf26e6bc9683 100644
--- a/webrtc/modules/audio_coding/codecs/audio_decoder.cc
+++ b/webrtc/modules/audio_coding/codecs/audio_decoder.cc
@@ -82,11 +82,6 @@ bool AudioDecoder::PacketHasFec(const uint8_t* encoded,
   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.