Do not enable spacewalk-service in runlevel 4

Runlevel 4 is not used in SUSE Linux and this causes some errors, related to "insserv" when the services are setup. The issue does not affects the Spacewalk installation, however it outputs error to STDERR where other tools reacts critically.

Seems like RHEL does not use this level too and runlevel 4 was left for historical reasons. We suggest to remove it.