debian: Drop ExecPreStart hack

[tintou]

All the other display managers have been migrated since a while now.

[lo1ol]

I faced some problem with this check too, in project that install custom greeter on system

systemd control singularity of the display manager's using alias "display-manager":

systemctl status display-manager

# disable previous one
systemctl disable display-manager
systemctl enable lightdm

# systemd controls that you can't enable several display-managers
# this command fails:
systemctl enable gdm

So, this check really looks like over check