Remove extra branch on Clusters::Platforms::Kubernetes#predefined_variables

Requires https://gitlab.com/gitlab-org/gitlab-ce/issues/54318 to be completed first.

Problem

else conditional was added on Clusters::Platforms::Kubernetes#predefined_variables to ensure compatibility with those Clusters that do not have Clusters::KubernetesNamespace relationship for some reason.

After background migration on 11.5 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22433, we need to remove it, as it might not be necessary

Proposal