RDMA/mlx5: Remove pd from struct mlx5_core_mkey

There is no read of mkey->pd, only write. Remove it.

Signed-off-by: Aharon Landau <aharonl@nvidia.com>
Reviewed-by: Shay Drory <shayd@nvidia.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>