[virtualdekstops] Drop multi-head from VirtualDesktopManagement

Kwin's VirtualDesktop saved a different configuration per X screen. This
messes up with a recent refactor of our startup as now we load
virtualdesktops before the X11 screen number is set and we load from the
group "Desktops-screen--1" then save to another place.

For a wayland session it doesn't make sense to have such a strong X
tie-in. We may as well just tidy this up.

Multihead has been deprecated for a decade. There is absolutely no way
Plasma supports it. Worst case if someone magically is using mutlihead
they just get syncronised virtual desktops.

BUG: 424571