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