gdm: remove old workaround for conflicts with getty

- There used to be a conflict between gdm and getty both trying to
access tty1
- This conflict was fixed by running gdm on tty7 instead

nixos/modules/services/x11/display-managers/gdm.nix

···
       "rc-local.service"
       "systemd-machined.service"
       "systemd-user-sessions.service"
-      "[e-mail]"
     ];
 
     systemd.services.display-manager.serviceConfig = {