debian/rules: remove unnecessary rules override

Replace override of dh_installinit with one that override just dh_systemd_start. Since we have --with systemd, we should not override an unrelated dh step to have a systemd unit enabled.

Ohter than simplify it, this fixes duplicated systemd section in the postinst script.

And while we're at it, add dh-systemd to B-Ds for a good measure.