io_uring: remove uring_cmd cookie

No more users of this field.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20231130215309.2923568-5-kbusch@meta.com


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