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_api.c

@@ -206,8 +206,3 @@ decoder_data(struct decoder *decoder,
 
 	return DECODE_COMMAND_NONE;
 }
-
-void decoder_flush(mpd_unused struct decoder *decoder)
-{
-	ob_flush();
-}