Remove add_on_purchases from AvailableServices

What does this MR do and why?

This removes add_on_purchases from AvailableServices, which was replaced in !191493 (merged) behind a FF, which I also remove here.

This is part of a larger refactor, see Replace methods that check for AddOnPurchases (#542422 - closed)

Not user-facing.

References

• #542422 (closed)

How to set up and validate locally

This should not result in any visible or testable changes.

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Related to #542422 (closed)