Now that QT_QUICK_CONTROLS_1_STYLE is not set in startkde anymore, drop the workaround for QGuiApplication crashes. Qt already takes care of setting the QQC1 style to Desktop for QApplications, so there's no reason why this should need to be set explicitely. If someone still does, we can assume that they know what they're doing and the consequences thereof, including QGuiApplication crashes.
Depends on D7998.