Remove sh -c from headless server systemd file

See https://github.com/jamulussoftware/jamulus/issues/1515 and https://github.com/jamulussoftware/jamulus/discussions/3183#discussioncomment-7158979

I don't think it's needed anymore and just adds complexity.