Revert "units: use `systemctl exit` to kill the user manager (#8648)"

This reverts commit add384dd.

The problem is that using systemctl to send the signal is nice, but requires
a working dbus. #10414 fixes this in a nicer way.

See https://github.com/systemd/systemd/issues/10414#issuecomment-438656395,
https://bugzilla.redhat.com/show_bug.cgi?id=1664491.