Follow-up from "Remove cache from BaseAvailableServiceData"

The following discussion from !159232 (merged) should be addressed:

We removed the cache of USER_ADD_ON_ASSIGNMENT_CACHE_KEY in !159232 (merged) , we should follow-up on this by either re-introducing this cache or by removing the deletion of the cache in the after_save in ee/app/models/gitlab_subscriptions/user_add_on_assignment.rb

Also, any related areas that might be deleting the cache with the key USER_ADD_ON_ASSIGNMENT_CACHE_KEY = 'addon-assignments:user-%{user_id}'.