systemd: remove redundant configure options

* --with-dbuspolicydir, --with-dbussessionservicedir, and
   --with-dbussystemservicedir are no longer needed since these are
   defaults in recent releases.

 * --disable-dbus has no effect when combined with --disable-tests. The
   option itself only applies to tests, not the runtime.

Signed-off-by: Gabe Evans <gabe@hashrabbit.co>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>