md: clean up openers check in do_md_stop() and md_set_readonly()

Before stopping or setting readonly, mddev_set_closing_and_sync_blockdev()
is always called to check the openers. So no longer need to check it again
in do_md_stop() and md_set_readonly(). Clean it up.

Signed-off-by: Li Nan <linan122@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240226031444.3606764-9-linan666@huaweicloud.com