[kcms/desktoppaths] Drop autostart path setting
The path for autostart files is an implementation detail that the user doesn't care about.
It also does not match the XDG autostart spec that defines the autostart dir relative to XDG_DATA_DIRS.
Plasma also does not use this directly. It is only used when migrating KDE4 config (https://invent.kde.org/plasma/plasma-workspace/-/blob/master/startkde/plasma-session/startup.cpp#L385)