spamassassin: don't create user/group debian-spamd

These are created by the (trusty) package's postinst and their existence
from puppet is making the postinst unhappy and the package
uninstallable.

As the creation is both redundant and hurtful, remove these User/Group
definitions.

Change-Id: I1fee4bddc46b9b89cbf827bd9988645a67e6ec01