Provisional revert of r89648: "Another try at fixing bug 93 "tilde signatures inside...

Provisional revert of r89648: "Another try at fixing bug 93 "tilde signatures inside nowiki tags sometimes get expanded (<includeonly><nowiki>~~~~</nowiki></includeonly>)""

This seems to disable PST for <includeonly>...</includeonly> in addition to the <includeonly><nowiki>...</nowiki></includeonly> that the bug is about.
This was caught by a parser test case, but the test case was changed to match... seems like the wrong change?

Also failed to update Preprocessor_HipHop.hphp to match the other two preprocessor classes.