Temporarily (I hope) disable flattening of IN/EXISTS sublinks that are within

the ON clause of an outer join.  Doing so is semantically correct but results
in de-optimizing queries that were structured to take advantage of the sublink
style of execution, as seen in recent complaint from Kevin Grittner.  Since
the user can get the other behavior by reorganizing his query, having the
flattening happen automatically is just a convenience, and that doesn't
justify breaking existing applications.  Eventually it would be nice to
re-enable this, but that seems to require a significantly different approach
to outer joins in the executor.