Remove 'fallback_default_namespace' method on Clusters::Platform::Kubernetes

This is a follow-up issue of https://gitlab.com/gitlab-org/gitlab-ce/issues/51716

fallback_default_namespace method was added to ensure compatibility with Clusters that were not migrated by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22433#note_114873891

After 11.5 this should not be necessary