Remove ArcanistWorkflow::setWorkingCopy
Summary: Fixes T9583. setWorkingCopy doesn't actually work, so eliminate it.
Test Plan: arc unit --everything
Reviewers: chad, epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin
Maniphest Tasks: T9583
Differential Revision: https://secure.phabricator.com/D14293