Simplify ChromeActions.isInPrivateBrowsing to not rely on the global PB service

This addresses the comments that I had on #2481.  PrivateBrowsingUtils.isWindowPrivate works fine without throwing in all Firefox versions now, and there is no need for the nsIPrivateBrowsingService fallback any more.  Also, it doesn't make sense to cache the result here since isWindowPrivate is not an expensive call.  This patch heavily simplifies isInPrivateBrowsing.