mlxsw: spectrum_router: Return void from deletion functions

There is no point in returning a value from function whose return value
is never checked.

Even if the return value was checked, there wouldn't be anything to do
about it, as these functions are either called from error or deletion
paths.

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>