Remove redundant contributable_target_types method
What does this MR do and why?
This is a follow-up from !138688 (comment 1800814268).
Because we created a new scope epic_contributions
in EE::Event.
It will also fix the inconsistency of not using a feature flag actor here
whereas the actor is being used in other places where
the epic_events_on_contributions_calendar
feature flag is used.
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Related to #382920 (closed)