Remove ad hoc README

It's unlikely that people will see this file, so it's kind of
pointless.

Changed files
-1
nixos
modules
services
x11
desktop-managers
-1
nixos/modules/services/x11/desktop-managers/README
···
-
Each desktop manager should explicitly specify a value for `services.xserver.displayManager.desktopManagerHandlesLidAndPower`.