Bug 1611645 - Don't set documentElement.title in private windows. r=Gijs

Please see https://bugzilla.mozilla.org/show_bug.cgi?id=1611645#c9 for a
summary.

Differential Revision: https://phabricator.services.mozilla.com/D76584

browser/base/content/browser.js

@@ -8235,7 +8235,6 @@ var gPrivateBrowsingUI = {
     // Adjust the window's title
     let docElement = document.documentElement;
     if (!PrivateBrowsingUtils.permanentPrivateBrowsing) {
-      docElement.title = docElement.getAttribute("title_privatebrowsing");
       docElement.setAttribute(
         "titlemodifier",
         docElement.getAttribute("titlemodifier_privatebrowsing")