1099168 - move %postun block inside pulp_server if block

Previously, the %postun was fenced only by %use_systemd. This caused strange
issues when importing the srpm to our build system, even though the srpm built
fine.

Instead, fence %postun by %pulp_server as well.

https://bugzilla.redhat.com/show_bug.cgi?id=1099168