RDMA/rxe: Fix useless copy in send_atomic_ack

In send_atomic_ack() in rxe_resp.c there is code copying ack_pkt into the
skb->cb[]. This doesn't do anything useful because the cb[] is not used in
the transmit path by the rxe driver.

Remove this code.

Fixes: 4c93496f

 ("IB/rxe: do not copy extra stack memory to skb")
Link: https://lore.kernel.org/r/20210618045742.204195-2-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearson@hpe.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>