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/mpc_plugin.c

@@ -231,8 +231,6 @@ mpc_decode(struct decoder *mpd_decoder, struct input_stream *inStream)
 			     replayGainInfo);
 	}
 
-	decoder_flush(mpd_decoder);
-
 	freeReplayGainInfo(replayGainInfo);
 
 	return 0;