Revert "kcms/autostart: Show correct icons for scripts"

This reverts commit bb51b556.

That code uses QFileIconProvider, which uses
QGuiApplicationPrivate::platformTheme(), which is null because
startplasma(-x11 in my case) only instantiates a QCoreApplication.
I do have some old-style startup scripts, so the whole session
crashed and I got back to the X session manager.