decoder: automatically flush the output buffer after decoder exits

A decoder_flush() invocation was missing in the FLAC plugin, resulting
in casual assertion failures due to a wrong assumption about the last
chunk's audio format.  It's much easier to remove that decoder_flush()
function and make the decoder thread call ob_flush().

src/decoder/mp4_plugin.c

@@ -298,8 +298,6 @@ mp4_decode(struct decoder *mpd_decoder, struct input_stream *inStream)
 	if (decoder_get_command(mpd_decoder) == DECODE_COMMAND_SEEK && seeking)
 		decoder_command_finished(mpd_decoder);
 
-	decoder_flush(mpd_decoder);
-
 	return 0;
 }