RDMA/verbs: Remove restrack entry from XRCD structure

XRCD object is not implemented in the restrack, so lets remove it.

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>