iptables: remove useless ip6tables VM rules

This rule isn't restored when the Kokoro VM is restarted, so it's not doing
anything. And the problem it was meant to solved is instead addressed by
https://github.com/google/gvisor/pull/3207.

PiperOrigin-RevId: 321026846

tools/vm/ubuntu1604/25_docker.sh

@@ -60,6 +60,3 @@ cat > /etc/docker/daemon.json <<EOF
     "ipv6": true
 }
 EOF
-# Docker's IPv6 support is lacking and does not work the same way as IPv4. We
-# can use NAT so containers can reach the outside world.
-ip6tables -t nat -A POSTROUTING -s 2001:db8:1::/64 ! -o docker0 -j MASQUERADE