Remove :kubernetes_namespace_per_environment feature flag
Follow up to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30711, remove the feature flag that controls separate namespaces per environment once we have tested it works.
Follow up to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30711, remove the feature flag that controls separate namespaces per environment once we have tested it works.