ceph: remove call to bdi_unregister

bdi_destroy already does all the work, and if we delay freeing the
anon bdev we can get away with just that single call.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>