Remove SdpAudioFormat's default constructor

webrtc/modules/audio_coding/codecs/audio_format.h

 /*
  *  Copyright (c) 2016 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.
  */
 
 #ifndef WEBRTC_MODULES_AUDIO_CODING_CODECS_AUDIO_FORMAT_H_
 #define WEBRTC_MODULES_AUDIO_CODING_CODECS_AUDIO_FORMAT_H_
 
 #include <map>
 #include <ostream>
 #include <string>
 #include <utility>
 
 namespace webrtc {
 
 // SDP specification for a single audio codec.
 // NOTE: This class is still under development and may change without notice.
 struct SdpAudioFormat {
   using Parameters = std::map<std::string, std::string>;
 
-  // TODO(kwiberg): Get rid of the default constructor when rtc::Optional no
-  // longer requires it.
-  SdpAudioFormat();
   SdpAudioFormat(const SdpAudioFormat&);
   SdpAudioFormat(SdpAudioFormat&&);
   SdpAudioFormat(const char* name, int clockrate_hz, int num_channels);
   SdpAudioFormat(const char* name,
                  int clockrate_hz,
                  int num_channels,
                  Parameters&& param);
   ~SdpAudioFormat();
 
   SdpAudioFormat& operator=(const SdpAudioFormat&);
   SdpAudioFormat& operator=(SdpAudioFormat&&);
 
   std::string name;
   int clockrate_hz;
   int num_channels;
   Parameters parameters;
   // Parameters feedback_parameters; ??
 };
 
 void swap(SdpAudioFormat& a, SdpAudioFormat& b);
 std::ostream& operator<<(std::ostream& os, const SdpAudioFormat& saf);
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_AUDIO_CODING_CODECS_AUDIO_FORMAT_H_