Remove PhabricatorBotDifferentialNotificationHandler reference in example_config.json for phabot
Summary: D9468 removed deprecated PhabricatorBotDifferentialNotificationHandler. So, lets remove it from the example json.
Test Plan: Verify json is valid, launch a phabot instance with it
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D9477