Remove the `PhabricatorBotDifferentialNotificationHandler` class.

Summary: This class has been deprecated for a while now (see rPdad7c65bf56384480be7c18e02fdc01ea67cf1ff). It should be safe to remove.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9468