Re-enable Parsoid on private wikis

The Parsoid PHP extension is now needed by VisualEditor to run, so the
special case "efficiency" of not loading Parsoid for private wikis no
longer works. The Parsoid extension already stops itself from inserting
private entries into the Varnish cache so this is not an issue.

Change-Id: I5265c408443212536a5ed96d910caba50c22e767