Remove parts as well in prerm

templates/debian/prerm

@@ -9,6 +9,8 @@ case "$1" in
     # XXX-Cedric: there are smarter ways to do that
     upgrade|failed-upgrade)
         rm -rf /opt/slapos/eggs
+        rm -rf /opt/slapos/develop-eggs
+        rm -rf /opt/slapos/parts
     ;;
 esac