RDMA/rxe: Remove qp->grp_lock and qp->grp_list

Since it is no longer required to cleanup attachments to multicast
groups when a QP is destroyed qp->grp_lock and qp->grp_list are
no longer needed and are removed.

Link: https://lore.kernel.org/r/20220127213755.31697-7-rpearsonhpe@gmail.com


Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>