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

src/applications/phriction/storage/PhrictionDocument.php

@@ -194,9 +194,6 @@ final class PhrictionDocument extends PhrictionDAO
     return false;
   }
 
-  public function shouldShowSubscribersProperty() {
-    return true;
-  }
 
 /* -(  PhabricatorApplicationTransactionInterface  )------------------------- */