Re-add field-level validation in hook_field_attach_validate()

Commit fd21608 introduced a working field-level validator that validates taxonomy field value changes based on the user's create permissions, using hook_field_attach_validate().

However, our implementation of this hook was returning errors even after our form-level validator corrected disallowed changes. I suspect there was a caching-related issue. In any case, several attempted fixes resulted in one or both of the validation steps malfunctioning.

I have temporarily commented out this hook implementation, because it is far more common for end users to update an entity via a form than with the API. However, once our validation is more robust, we should fix this hook.