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/ffmpeg_plugin.c

@@ -314,10 +314,7 @@ static int ffmpeg_decode_internal(BasePtrs *base)
 		}
 	} while (decoder_get_command(decoder) != DECODE_COMMAND_STOP);
 
-	decoder_flush(decoder);
-
 	DEBUG("decoder finish\n");
-
 	return 0;
 }