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