Remove Phabricator reference from the linter description of ArcanistXHPASTLinter

Summary: The ArcanistXHPASTLinter is no longer too specific to Phabricator (Phabricator-specific XHPAST lints generally live in ArcanistPhutilXHPASTLinter) and should be better suited for general purpose usage.

Test Plan: N/A

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9892