RDMA/core: Remove unused ib cache functions

Now that all users have been converted to use the version of these APIs
that returns a gid_attr pointer we can delete the old entry points.

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