Revert incorrect memory-conservation hack in inheritance_planner().

This reverts commit d1001a78 of 2010-12-05,
which was broken as reported by Jeff Davis.  The problem is that the
individual planning steps may have side-effects on substructures of
PlannerGlobal, not only the current PlannerInfo root.  Arranging to keep
all such side effects in the main planning context is probably possible,
but it would change this from a quick local hack into a wide-ranging and
rather fragile endeavor.  Which it's not worth.