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

GDM now specifies ordering between `plymouth-quit` and `display-manager`:
9be53210

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
9be53210 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).