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Remove KUBE_API_VERSIONS

Fixes https://github.com/kubernetes/kubernetes/issues/63102

KUBE_API_VERSIONS is an attempt to control the available serialization of types. It pre-dates the idea that we'll have separate schemes, so it's not a thing that makes sense anymore.

Server-side we've had a very clear message about breaks in the logs for a year "KUBE_API_VERSIONS is only for testing. Things will break.".

Client-side it became progressively more broken as we moved to generic types for CRUD more than a year ago. What is registered doesn't matter when everything is unstructured.

We should remove this piece of legacy since it doesn't behave predictable server-side or client-side.

@smarterclayton @lavalamp
@kubernetes/sig-api-machinery-bugs 

```release-note
KUBE_API_VERSIONS is no longer respected.  It was used for testing, but runtime-config is the proper flag to set.
```