Remove XUL overlays from Torbutton

XUL overlays (and XUL itself, actually) have been discouraged for years, and are already being removed outright from Firefox:

https://bugzilla.mozilla.org/show_bug.cgi?id=1426763 https://bugzilla.mozilla.org/show_bug.cgi?id=1448162 https://bugzilla.mozilla.org/show_bug.cgi?id=1449791 https://bugzilla.mozilla.org/show_bug.cgi?id=1450753