IB/rxe: Removed GID add/del dummy routines

rxe driver's add_gid() and del_gid() callbacks are doing simple
checks which are already done by the ib core before invoking these
callback routines.
Therefore, code is simplified to skip implementing add_gid() and
del_gid() callback functions.
They are only invoked by ib_core if they are implemented.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>