Revert "block: take bd_mutex around delete_partitions in del_gendisk"

hulk inclusion
category: bugfix
bugzilla: 55097
CVE: NA

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This reverts commit 10097449.

The patch set for partition symlink cleanup will introduce
deadlock for nbd, loop and xen-blkfront driver, so revert it.
Reviewed-by: Jason Yan <yanaijie@huawei.com>

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>