IB/qib: Delete struct qib_ivdev.qp_rnd

I was checking the field to see if it needed the full get_random_bytes()
and discovered it's unused.

Only compile-tested, as I don't have the hardware, but I'm still pretty
confident.

Link: https://lore.kernel.org/r/202003281643.02SGh6eG002694@sdf.org


Signed-off-by: George Spelvin <lkml@sdf.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>