Remove deprecated `PhabricatorIRCBot` class.

Summary: This class has been deprecated for a while (see rP574bc3ba31cca2767bafe7844d7f854d90d6be1c). It should be safe to remove now.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9469