Remove getPHID() from PhabricatorMentionableInterface

Summary: Some versions of PHP aren't very happy about both interfaces and superclasses defining a method. Just remove it from MentionableInterface and leave it as implicit.

Auditors: btrahan

src/applications/transactions/interface/PhabricatorMentionableInterface.php

@@ -5,8 +5,4 @@
  * - actually TYPE_EDGE transactions that add "mentioned" edges - to the
  * implementing object.
  */
-interface PhabricatorMentionableInterface {
+interface PhabricatorMentionableInterface {}
-
-  public function getPHID();
-
-}