wireguard: Don't use fwmarks for the gateways

wireguard: Don't use fwmarks for the gateways

This slightly conflicts with the reverse path filter which does not seem
to consider the mark and therefore does not resolve to the correct route.

There is not too much benefit of using the mark, except its elegance, a
more accurate lookup and that we were hiding a direct route to the
gateway from the clients.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>