song/DetachedSong: copy the AudioFormat from LightSong

Enables the "Format" row in "playlistinfo" responses.

https://github.com/MusicPlayerDaemon/MPD/issues/1094

src/db/plugins/simple/DirectorySave.cxx

@@ -168,15 +168,12 @@ directory_load(TextFile &file, Directory &directory)
 				throw FormatRuntimeError("Duplicate song '%s'", name);
 
 			std::string target;
-			auto audio_format = AudioFormat::Undefined();
 			auto detached_song = song_load(file, name,
-						       &target,
-						       &audio_format);
+						       &target);
 
 			auto song = std::make_unique<Song>(std::move(detached_song),
 							   directory);
 			song->target = std::move(target);
-			song->audio_format = audio_format;
 
 			directory.AddSong(std::move(song));
 		} else if ((p = StringAfterPrefix(line, PLAYLIST_META_BEGIN))) {