Remove SdpAudioFormat's default constructor

webrtc/modules/audio_coding/codecs/audio_format.h

Index: webrtc/modules/audio_coding/codecs/audio_format.h
diff --git a/webrtc/modules/audio_coding/codecs/audio_format.h b/webrtc/modules/audio_coding/codecs/audio_format.h
index 61c0dd9f6fa127c32c3fd075c71c10b993419ff9..9b971778f8fff42aa04c170688aa841ce022eaf9 100644
--- a/webrtc/modules/audio_coding/codecs/audio_format.h
+++ b/webrtc/modules/audio_coding/codecs/audio_format.h
@@ -23,9 +23,6 @@ namespace webrtc {
 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);