As per `man systemd.path`:
> When a service unit triggered by a path unit terminates
> (regardless whether it exited successfully or failed),
> monitored paths are checked immediately again,
> **and the service accordingly restarted instantly**.
Thus the existence of the path unit made it impossible to stop the
wireguard service using e.g.
systemctl stop wireguard-wg0.service
Systemd path units are not intended for program inputs such
as private key files.
This commit simply removes this usage; the private key is still
generated by the `generateKeyServiceUnit`.
d344dccf3d