block: get rid of q->softirq_done_fn()

With the legacy path gone, all we do is funnel it through the
mq_ops->complete() operation.

Tested-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>