Fixes <https://issues.guix.gnu.org/44944>.
* gnu/services/xorg.scm (%gdm-activation): Delete variable.
(gdm-service-type): De-register it.
* gnu/services/desktop.scm (%gdm-file-system): New variable.
(gdm-file-system-service): Likewise.
(desktop-services-for-system): Use it.