R1:abb4c03b476d

Remove shouldShowSubscribersProperty() from SubscribableInterface

Summary:
Every caller returns `true`. This was added a long time ago for Projects, but projects are no longer subscribable.

I don't anticipate needing this in the future.

Test Plan: Grepped for this method.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15409