Don't make FK-based selectivity estimates in inheritance situations.

The foreign-key-aware logic for estimation of join sizes (added in commit
100340e2) blindly tried to apply the concept to rels that are actually
parents of inheritance trees.  This is just plain wrong so far as the
referenced relation is concerned, since the inheritance scan may well
produce lots of rows that are not participating in the constraint.  It's
wrong for the referencing relation too, for the same reason; although on
that end we could conceivably detect whether all members of the inheritance
tree have equivalent FK constraints pointing to the same referenced rel,
and then proceed more or less as we do now.  But pending somebody writing
code to do that, we must disable this, because it's producing completely
silly estimates when there's an FK linking the heads of inheritance trees.

Per bug #14404 from Clinton Adams.  Back-patch to 9.6 where the new
estimation logic came in.

Report: <20161028200412.15987.96482@wrigleys.postgresql.org>