In GDM, when uninstalling it sets /etc/X11/default-display-manager to the first available display manager *package name*. It should set it to the full path to the display manager executable.
In LightDM, when uninstalling it doesn't change /etc/X11/default-display-manager, so it is set to "/usr/sbin/lightdm" which is no longer installed. It should set it to the full path to the first available display manager executable.
What exactly happens:
In GDM, when uninstalling it sets /etc/X11/ default- display- manager to the first available display manager *package name*. It should set it to the full path to the display manager executable.
In LightDM, when uninstalling it doesn't change /etc/X11/ default- display- manager, so it is set to "/usr/sbin/lightdm" which is no longer installed. It should set it to the full path to the first available display manager executable.