RDMA/restrack: Remove unimplemented XRCD object

Resource tracking of XRCD objects is not implemented in current
version of restrack and hence can be removed.

Fixes: 02d8883f ("RDMA/restrack: Add general infrastructure to track RDMA resources")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>