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/mod_plugin.c

@@ -210,8 +210,6 @@ static int mod_decode(struct decoder * decoder, char *path)
 			     total_time, 0, NULL);
 	}
 
-	decoder_flush(decoder);
-
 	mod_close(data);
 
 	MikMod_Exit();