WIP: When iterating over PrintObject's regions, use the region count

by PrintObject::region_volumes. This is due to the way Print::apply()
works, it does not invalidate an existing PrintObject if a new region
is added to the print.

xs/xsp/Print.xsp

@@ -44,8 +44,6 @@ _constant()
     int region_count()
         %code%{ RETVAL = THIS->print()->regions().size(); %};
 
-    int region_volumes_count()
-        %code%{ RETVAL = THIS->region_volumes.size(); %};
     Ref<Print> print();
     Ref<ModelObject> model_object();
     Ref<StaticPrintConfig> config()