systemctl: remove --follow option

Given that "journalctl -u" exists now there's no need to duplicate this
functionality in systemctl, so let's drop this, especially given that it
always felt a bit awkward to overload "-f" to both --force and --follow,
and to have continues output with a status header for this.

        systemctl status -f avahi-daemon

now becomes:

        journalctl -fu avahi-daemon

Which is shorter and a lot less redundant.