Get rid of duplicate child RTE for a partitioned table.

Get rid of duplicate child RTE for a partitioned table.

We've been creating duplicate RTEs for partitioned tables just
because we do so for regular inheritance parent tables.  But unlike
regular-inheritance parents which are themselves regular tables
and thus need to be scanned, partitioned tables don't need the
extra RTE.

This makes the conditions for building a child RTE the same as those
for building an AppendRelInfo, allowing minor simplification in
expand_single_inheritance_child.  Since the planner's actual processing
is driven off the AppendRelInfo list, nothing much changes beyond that,
we just have one fewer useless RTE entry.

Amit Langote, reviewed and hacked a bit by me

Discussion: https://postgr.es/m/9d7c5112-cb99-6a47-d3be-cf1ee6862a1d@lab.ntt.co.jp