Remove unused cluster configuration logic

Now that project-specific Kubernetes resources are created just-in-time (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29515), we can remove the functionality that creates resources as soon as the cluster is created.

• Remove Clusters::RefreshService - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31227
• Remove .missing_kubernetes_namespace and friends - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25725
• Remove workers and queues (Must be done in 13.0) - !30695 (merged)