Revert "blk-mq, percpu_ref: implement a kludge for SCSI blk-mq stall during probe"

This reverts commit 0a30288da1aec914e158c2d7a3482a85f632750f, which
was a temporary fix for SCSI blk-mq stall issue.  The following
patches will fix the issue properly by introducing atomic mode to
percpu_ref.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Kent Overstreet <kmo@daterainc.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christoph Hellwig <hch@lst.de>