Reduce the cost of planning deeply-nested views.

Joel Jacobson reported that deep nesting of trivial (flattenable)
views results in O(N^3) growth of planning time for N-deep nesting.
It turns out that a large chunk of this cost comes from copying around
the "subquery" sub-tree of each view's RTE_SUBQUERY RTE.  But once we
have successfully flattened the subquery, we don't need that anymore,
because the planner isn't going to do anything else interesting with
that RTE.  We already zap the subquery pointer during setrefs.c (cf.
add_rte_to_flat_rtable), but it's useless baggage earlier than that
too.  Clearing the pointer as soon as pull_up_simple_subquery is done
with the RTE reduces the cost from O(N^3) to O(N^2); which is still
not great, but it's quite a lot better.  Further improvements will
require rethinking of the RTE data structure, which is being considered
in another thread.

Patch by me; thanks to Dean Rasheed for review.

Discussion: https://postgr.es/m/797aff54-b49b-4914-9ff9-aa42564a4d7d@www.fastmail.com

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