Remove istio-cni.yaml from pre-gl-gke-2 kustomization
What
Remove istio-cni.yaml kustomization since has been moved to the environment shared path.
Why
Related Issue: https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/24157
Remove istio-cni.yaml kustomization since has been moved to the environment shared path.
Related Issue: https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/24157