Allow ArcanistPhutilLibraryLinter to run with an out-of-date XHPAST binary

Summary: There is no need to check if the XHPAST binary is available here because this linter does not actually parse PHP, it only considers the library map.

Test Plan: Removed the XHPAST binary and ran arc lint on a bunch of files.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11528