BACKPORT: block: simplify calling convention of elv_unregister_queue()

Make elv_unregister_queue() a no-op if q->elevator is NULL or is not
registered.

This simplifies the existing callers, as well as the future caller in
the error path of blk_register_queue().

Also don't bother checking whether q is NULL, since it never is.

Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220124215938.2769-2-ebiggers@kernel.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>

(cherry picked from commit f5ec592d)

Conflicts:
	block/blk-sysfs.c

Bug: 207390665
Change-Id: I9e4226b7241755ca22dd9687b084e0077885d222
Signed-off-by: Eric Biggers <ebiggers@google.com>