Remove the deprecated PhabricatorIRCProtocolHandler class.

Summary: This class has been deprecated for a while now (see rP0a8b0d1392bd79b4e88fbf910b176c960d57b4b4). 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/D9467