block: remove the nr_task field from struct io_context

Nothing ever looks at ->nr_tasks, so remove it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20211209063131.18537-2-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>