RimColorLegendCollection::deleteColorLegend erased the default-legend map
entry before looking the legend up, so findDefaultLegendForResult always
returned null and the old legend was never removed from m_customColorLegends.
Each set_discrete_property_category_names call for the same property left an
orphaned legend behind and appended a new one.
Look up the legend before erasing the map key, and guard against deleting a
standard legend that may have been registered as a default.