IB/qedr: Remove GID add/del dummy routines

qedr 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.

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