Do not remove /etc/default/puppet

We are not shipping /etc/default/puppet anymore, however it is still
used by the initscript and potentially sourced by the systemd unit file
(see #778891). Since we have no way to automatically migrate user
changes to puppet.conf [agent] flags, we shouldn't remove the conffile
if it exists.

Dropping the call to dpkg-maintscript-helper means that we can remove
puppet.{postinst,postrm,preinst} since they are not used for anything
else.