Prevent join removal from removing the query's result relation.

This was not something that required consideration before MERGE
was invented; but MERGE builds a join tree that left-joins to the
result relation, meaning that remove_useless_joins will consider
removing it.  That should generally be stopped by the query's use
of output variables from the result relation.  However, if the
result relation is inherited (e.g. a partitioned table) then
we don't add any row identity variables to the query until
expand_inherited_rtentry, which happens after join removal.

This was exposed as of commit 3c569049b, which made it possible
to deduce that a partitioned table could contain at most one row
matching a join key, enabling removal of the not-yet-expanded
result relation.  Ooops.

To fix, let's just teach join_is_removable that the query result
rel is never removable.  It's a cheap enough test in any case,
and it'll save some cycles that we'd otherwise expend in proving
that it's not removable, even in the cases we got right.

Back-patch to v15 where MERGE was added.  Although I think the
case cannot be reached in v15, this seems like cheap insurance.

Per investigation of a report from Alexander Lakhin.

Discussion: https://postgr.es/m/36bee393-b351-16ac-93b2-d46d83637e45@gmail.com

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