[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 virtual desktops 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 is using mutlihead they just get synchronised virtual desktops.