parent query's EState. Now that there's a single flat rangetable for both
the main plan and subplans, there's no need anymore for a separate EState,
and removing it allows cleaning up some crufty code in nodeSubplan.c and
nodeSubqueryscan.c. Should be a tad faster too, although any difference
will probably be hard to measure. This is the last bit of subsidiary
mop-up work from changing to a flat rangetable.