With o10-2.2.3 enforced rxe_drop_all_mcast_groups is completely
unnecessary. Remove it and references to it.

Link: https://lore.kernel.org/r/20220127213755.31697-6-rpearsonhpe@gmail.comSigned-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>