Adjust setRelhassubclassInRelation() to not perform actual heap_update

Adjust setRelhassubclassInRelation() to not perform actual heap_update
when the pg_class.relhassubclass value is already correct.  This should
avoid most cases of the 'tuple concurrently updated' problem that
Robert Creager recently complained about.  Also remove a bunch of dead
code in StoreCatalogInheritance() --- it was still computing the complete
list of direct and indirect inheritance ancestors, though that list has
not been needed since we got rid of the pg_ipl catalog.