Simplify SubscriptionsHelper API

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR is just polishing the SubscriptionsHelper from redundancy in the add-on-purchase-related methods. Exposing a method that it's not used has no benefit and could lead to inconsistent use of the API.

Please note that this feature is still under development and behind FF (#322582 (closed)) so not user-facing yet.

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