state/apiserver/usermanager: removed unused dummy authenticators

This PR removes the unused canRead/canWrite authenticators from this package. The justification is as follows.

* The authentators were hard coded to always authenticate whatever value was passed, _even if it was not valid_

* If the authenticators did validate their data they would find that arg.Tag passed to them was commonly empty, which is bad for two reasons

1. "" is not a valid API tag
2. "" is the synonym for "admin"

* Lastly, the tests continue to pass without them ...