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