net/mlx5: Remove unused structs

Remove structs which are no longer used in the driver:
  mlx5dr_cmd_qp_create_attr
  mlx5_fs_dr_ns
  mlx5_pas

Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>