Remove an unused variable

Summary: Ref T7604. Remove an unused variable from ArcanistDiffWorkflow.

Test Plan: This diff, I guess.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7604

Differential Revision: https://secure.phabricator.com/D12972