blk-mq: remove __blk_execute_rq_nowait

blk-mq: remove __blk_execute_rq_nowait

We don't want to plug for synchronous execution that where we immediately
wait for the request.  Once that is done not a whole lot of code is
shared, so just remove __blk_execute_rq_nowait.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20220524121530.943123-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>