fake client-go: un-deprecate NewSimpleClientset

NewSimpleClientset was marked as deprecated when NewClientset was
introduced. This has caused some confusion:
- Not all packages have NewClientset (kubernetes/kubernetes#135980).
- Tests that work with NewSimpleClientset fail when
  switched to NewClientset (kubernetes/kubernetes#136327)
  because of missing CRD support (kubernetes/kubernetes#126850).

It doesn't seem burdensome to keep NewSimpleClientset around forever. Some unit
tests may even prefer to use it when they don't need server-side apply (less
overhead). Therefore there is no need to deprecate it.

This avoids churn in the eco system because contributors no longer create PRs
"because the linter complains about the usage of a deprecated function".

Kubernetes-commit: bd399917375d2a1d77c04f8dfd67a67301af9721

Author: pohly