Prevent entity form controllers from being cached twice (in $form and $form_state)

Spin-off from #1499596-96: Introduce a basic entity form controller → :

I noticed that due to $controller->form_state the form state gets serialized *into itself* when it is cached. I've discussed that with plach and we agreed to remove $controller->form_state for now and just pass it on to respective methods always. We may want to simplify the getEntity() calls to do not require the passed on form-state again in the follow-up where we fix the controller being still duplicated to $form and $form_state.

To be completed.