I got rid of the useless pam_systemd by running "pam-config --delete
--systemd".
(See http://seifesrants.blogspot.de/2014/11/pamsystemd-on-server-wtf.html)
However, today I found that it magically reappeared and is spamming my system
logs for no good reason.
Checking, I noticed that systemd was updated lately.

Looking into the postinstall stuff, I found the following "gem":
postinstall scriptlet (using /bin/sh):
/usr/sbin/pam-config -a --systemd || :
/sbin/ldconfig
[...]

This unconditional adding is not really a good idea IMNSHO, if it is done at
all, it should only be done at initial install of systemd.