don't show parent properties for case types that aren't actual parents

`get_parent_types_and_contributed_properties` returns two things which are somewhat related but not intrinsically tied. The contributed properties returned here don't require that the case_type of the contributor refer to an actual parent case_type. The contributor only needs to open a case in a different module. 

This might merit splitting this function out into two, but I'm not sure of the best way to do.