Allow primary key attributes

[mike-burns]

This improvement went out too quickly, so let's undo it.

Lots of the work around it was great and we want to keep that, so this surgically snips out:

• The README describing the feature.
• The ActiveRecordValidator that implements the validation.
• The railtie code that loaded the validator.

It leaves the validation framework and the notification code.

[mike-burns]

I'd like to see this come back as a lint against the auto_populated? predicate.