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Remove experimental keystone authenticator

**What this PR does / why we need it**:

experimental-keystone-url and experimental-keystone-ca-file were always
experimental. So we don't need a deprecation period.
KeystoneAuthenticator was on the server side and needed userid/password
to be passed in and used that to authenticate with Keystone. We now
have authentication and authorization web hooks that can be used. There
is a external repo with a webook for keystone which works fine along
with the kubectl auth provider that was added in:
a0cebcb5

So we don't need this older style / hard coded / experimental code
anymore.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #

**Special notes for your reviewer**:

**Release note**:

```release-note
kube-apiserver: the experimental in-tree Keystone password authenticator has been removed in favor of extensions that enable use of Keystone tokens.
```