systemd.bbclass: drop postrm, use prerm instead

* Calling "systemctl disable" from postrm is too late and
  causes the following error:

| Failed to issue method call: No such file or directory

* Messing with $D is not needed in *rm scripts.

Signed-off-by: Andreas Oberritter <obi@opendreambox.org>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>