kmscon: reset ExecStart to allow override

The getty@.service unit already has an ExecStart so we cannot simply set a new
one in order to override it or we will get this error:

systemd[1]: getty@tty1.service: Service has more than one ExecStart= setting, which is only allowed for Type=oneshot services. Refusing.

Instead "reset" ExecStart by setting it to empty which is the systemd way of
doing it.