Additional_fields in User Update (provider) and User Create (provider) endpoints doesn't add extra User fields

Currently all customers (providers) can see and use `additional_fields` under User Update (provider) and User Create (endpoint) endpoints. However, those fields doesn't work as expected because this functionality can only be used by Red Hat 3scale admin system for the customer accounts.

The request is to hide `additional_fields`to User Update (provider) and User Create (provider) endpoints as it is misleading for our customers.