journal: don't remove the flushed flag when journald is stopped

journal: don't remove the flushed flag when journald is stopped

Due to the fact that systemd-journal-flush.service has
"Requires=systemd-journald.service", this service is stopped too when journald
is requested to do so.

However stopping systemd-journal-flush.service implies that journald
relinquishes /var hence implicitly switching back to the volatile storage
mode and removing /run/systemd/journal/flushed.

If journald is started afterwards, it will run in volatile storage mode
regardless of the value of 'Storage=' as it believes now that /var is not yet
ready (because the flushed flag is missing).

Because this flag is mainly an indication for journald that the initialization
of /var/log/journal (during the boot process) has been done,
systemd-journal-flush.service shouldn't be tied to the state of journald itself
but to the state of /var/log/journal, hence to the state of the system.

(cherry picked from commit d5ee050ffc9d413253932d9340ade8c8fb111092)