output: removed audio_output.convertAudioFormat

Instead of checking convertAudioFormat, we can simply check if
reqAudioFormat is defined.  This saves 4 bytes in the struct.

src/output_api.h

@@ -60,7 +60,6 @@ struct audio_output {
 
 	const struct audio_output_plugin *plugin;
 
-	int convertAudioFormat;
 	struct audio_format inAudioFormat;
 	struct audio_format outAudioFormat;
 	struct audio_format reqAudioFormat;