decoder: return void from decode() methods

The stream_decode() and file_decode() methods returned a boolean,
indicating whether they were able to decode the song.  This is
redundant, since we already know that: if decoder_initialized() has
been called (and dc.state==DECODE), the plugin succeeded.  Change both
methods to return void.

src/decoder/oggflac_plugin.c

@@ -284,17 +284,15 @@ static bool oggflac_try_decode(struct input_stream *inStream)
 	return ogg_stream_type_detect(inStream) == FLAC;
 }
 
-static bool
+static void
 oggflac_decode(struct decoder * mpd_decoder, struct input_stream *inStream)
 {
 	OggFLAC__SeekableStreamDecoder *decoder = NULL;
 	FlacData data;
-	bool ret = true;
 
 	init_FlacData(&data, mpd_decoder, inStream);
 
 	if (!(decoder = full_decoder_init_and_read_metadata(&data, 0))) {
-		ret = false;
 		goto fail;
 	}
 
@@ -329,8 +327,6 @@ oggflac_decode(struct decoder * mpd_decoder, struct input_stream *inStream)
 
 fail:
 	oggflac_cleanup(&data, decoder);
-
-	return ret;
 }
 
 static const char *const oggflac_Suffixes[] = { "ogg", "oga", NULL };