QDate(Time)?::toString(Qt::DateFormat, QCalendar) are misguided

The Qt::DateFormat members that implicate a locale are going away at Qt 6; and they're the only ones whose toString() behaviour currently involves a calendar. So adding the calendar parameter to those methods turns out to have been a mistake and needs to be undone and/or (since one of them was added at 5.14) deprecated in 5.15 ready for removal in 6.