blk-cgroup: remove the !bio->bi_blkg check in blkcg_bio_issue_check

This is purely a sanity check for grave programming errors.  Remove it
to simplify further work in this area.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>