nullb: remove leftover legacy request code

null_softirq_done_fn is only used for the blk-mq path, so remove the
other branch.  Also rename the function to better match the method name.

Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>