Don't install crontabs when using systemd timer units

systemd now has timer units that can replace cron jobs.
Such units are installed by sysstat if requested to do so on machines
supporting systemd. In this case a crontab should no longer be
installed else a redundant line of statistics appear at every interval
of time.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>