gitlab-runner-values.yaml.tftpl: Don't set KOKKURI_REGISTRY_INTERNAL
As of Kokkuri v2.3.0, KOKKURI_REGISTRY_INTERNAL is optional and
defaults to the value of KOKKURI_REGISTRY_PUBLIC (which is still set
here). Leaving KOKKURI_REGISTRY_INTERNAL unset makes it easier for
users to override KOKKURI_REGISTRY_PUBLIC without having to worry
about a lurking KOKKURI_REGISTRY_INTERNAL setting.