Remove ipv6 customization from template
https://github.com/openshift/router/pull/325 was merged to 4.7 (and 4.8) branch and included in the latest 4.7 stable release https://amd64.origin.releases.ci.openshift.org/releasestream/4-stable/release/4.7.0-0.okd-2021-08-22-163618
This means we can remove our IPv6 customization - actually even if we don't use the new image immediately I think we should do it ASAP so custom-ingress is ready for our next OKD upgrade.