[systemd] Cosmetic: Fix non-existant display-manager dependency

The graphical.target in Systemd-216 has a "Wants" dependency on display-manager.service which is a non-existent service. Consequently, the following message appears in the kernel log:

```
[    5.676728] systemd[1]: Cannot add dependency job for unit display-manager.service, ignoring: Unit display-manager.service failed to load: No such file or directory.
```

Removing this dependency eliminates this message.