systemd.bbclass: stop using SYSTEMDPN to rdepends addition

The systemd runtime dependency needs to be add dynamically onto each
package that has the postinst generate thus removing the SYSTEMDPN use
et all.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>