What's the concensus on desktop-managers auto-enabling display-managers mapped to where it would make sense?

Your commit msg at fc035da says

If plasma5 is used as desktop-manager use sdddm.
If gnome3 is used as desktop-manager use gdm.

However if i do services.xserver.desktopManager.plasma5.enable = true;
it's lightdm not sddm.

I think this "auto-enabling" would make sense to have as long as it never interferes with someone enabling a different display-manager or enabling multiple desktop-managers.

Since this "auto-enabling" is only for gnome3 and it interferes with peoples use in an obtrusive way I think it's fine to remove this here.

Auto-enabling needs more logic than I thought. So we can enable gdm automatically if someone doesn't enable any other display-manager and light-dm is only by default enabled. Same for sddm/plasma.

But the easy fix is first to remove the gdm activation and cherry-pick this commit also to 18.09.

cc @jtojnar @hedning

Hopefully not too many people depend on this yet.