Don't restart systemd-journal-flush

It only needs to be started during boot. Starting it at other times
shouldn't hurt, except that if systemd-journald is restarting at the
same time, the latter might not have a SIGUSR1 signal handler
installed yet, so it might be killed by systemd-journal-flush. (At
least that's my theory about the dead systemd-journald instances in
the build farm...)