Remove a parameter

Summary: D7271 added this line but the signature of function ArcanistWorkingCopyIdentity::newFromRootAndConfigFile(...) was never changed. I have no idea why it was added here and it's causing failures in facebook. I am just removing it now.

Test Plan: No idea how to test

Reviewers: wez, lifeihuang, JoelB, #blessed_reviewers, epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8432