[systemd] Fixed mce start dependency. JB#15511

With new systemd we don't have anymore start-user-session@USER.service
We could fix this by using 
Before=start-user-session.service
but we don't need any start dependency in mce as start-user-session.service has this
After=mce.service
that is all we need

Signed-off-by: Pekka Lundstrom <pekka.lundstrom@jollamobile.com>