f2fs: remove request_list check in is_idle()

This doesn't work on stacked devices, and it doesn't work on
blk-mq devices. The request_list is only used on legacy, which
we don't have much of anymore, and soon won't have any of.

Kill the check.

Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: linux-f2fs-devel@lists.sourceforge.net
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>