Remove 'User#duo_pro_add_on_available?' method

What does this MR do and why?

We no longer use this method. We should remove the definition and specs.

Instead, we use Cloud Connector abstraction (example for duo_chat):

::CloudConnector::AvailableServices.find_by_name(:duo_chat).allowed_for?(user)

The Cloud Connector abstraction aims to decouple the logic from the particular add-on names (anticipating new add-ons will be introduced in the future).

Follow-up to the discussion !158048 (comment 1980025157)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #470487 (closed)