remove custom auth from config schema

[sblair-metrostar]

Fixes #1934

Removes the custom auth provider option from the config schema. Includes unit tests to cover the upgrade path that was intended to handle the removal of this previously.

What does this implement/fix?

Put a x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

• Did you test the pull request locally?
• Did you add new tests?

Any other comments?

[iameskild]

LGTM! Thanks @sblair-metrostar!!