mlxsw: spectrum_router: Destroy RIF only based on its struct

Now that all the information to create a RIF is contained within the RIF
struct itself, we can also simplify the destruction logic.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>