md: cleanup pers->prepare_suspend()

pers->prepare_suspend() is not used anymore and can be removed.

Reverts following three commit:

 - commit 431e6125 ("md: export md_is_rdwr() and is_md_suspended()")
 - commit 3e00777d ("md: add a new api prepare_suspend() in
md_personality")
 - commit 868bba54 ("md/raid5: fix a deadlock in the case that reshape
is interrupted")

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231016100240.540474-1-yukuai1@huaweicloud.com