Remove the deprecated `ArcanistWorkCopyIdentity::getConfig` method.

Summary: This method has been deprecated for a while now (see rARC0d212ccf5a7ac5fc75d2d3036cb3a0bee220fd02). 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/D9466