blk-mq: remove WARN_ON(!q->elevator) from blk_mq_sched_free_requests

blk_mq_sched_free_requests() may be called in failure path in which
q->elevator may not be setup yet, so remove WARN_ON(!q->elevator) from
blk_mq_sched_free_requests for avoiding the false positive.

This function is actually safe to call in case of !q->elevator because
hctx->sched_tags is checked.

Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Yi Zhang <yi.zhang@redhat.com>
Fixes: c3e22192 ("block: free sched's request pool in blk_cleanup_queue")
Reported-by:  <syzbot+b9d0d56867048c7bcfde@syzkaller.appspotmail.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>