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