net/mlx5: Remove the flag MLX5_INTERFACE_STATE_SHUTDOWN

MLX5_INTERFACE_STATE_SHUTDOWN is not used in the code.

Fixes: 5fc7197d ("net/mlx5: Add pci shutdown callback")
Signed-off-by: Huy Nguyen <huyn@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>