Remove "commit hook mode" workarounds for ArcanistXHPASTLinter
Summary: Ref T7674. ArcanistXHPASTLinter has some workarounds for running in "commit hook" mode (which has since been removed). This linter should always have a working copy.
Test Plan: arc unit
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Subscribers: Korvin, epriestley
Maniphest Tasks: T7674
Differential Revision: https://secure.phabricator.com/D12956