nixos/plymouth: do not order `plymouth-quit` after `display-manager`

GDM now specifies ordering between `plymouth-quit` and `display-manager`:
9be5321097

This causes an ordering cycle between GDM and plymouth-quit which can result in
systemd breaking GDM:
```
plymouth-quit.service: Job display-manager.service/start deleted to break
                       ordering cycle starting with plymouth-quit.service/start
```

Not sure how often this triggers, as I've run my system with plymouth and
9be5321097 without any issues. But I did catch a VM doing this.

NOTE: I also tried to remove the ordering in GDM to see if plymouth managed to
live longer, but it didn't seem to help. So I opted to stick as close to
upstream (upstream GDM specifies ordering, but plymouth does not).

nixos/modules/system/boot/plymouth.nix

@@ -88,10 +88,7 @@ in
     systemd.services.plymouth-kexec.wantedBy = [ "kexec.target" ];
     systemd.services.plymouth-halt.wantedBy = [ "halt.target" ];
     systemd.services.plymouth-quit-wait.wantedBy = [ "multi-user.target" ];
-    systemd.services.plymouth-quit = {
-      wantedBy = [ "multi-user.target" ];
-      after = [ "display-manager.service" ];
-    };
+    systemd.services.plymouth-quit.wantedBy = [ "multi-user.target" ];
     systemd.services.plymouth-poweroff.wantedBy = [ "poweroff.target" ];
     systemd.services.plymouth-reboot.wantedBy = [ "reboot.target" ];
     systemd.services.plymouth-read-write.wantedBy = [ "sysinit.target" ];