Remove redundant null-treatment check in window function dedup.

Remove redundant null-treatment check in window function dedup.

Commit 25a30bbd423 (IGNORE NULLS / RESPECT NULLS for window functions)
made ExecInitWindowAgg() treat two otherwise-equal window functions as
duplicates only when their ignore_nulls settings also matched:

    if (i <= wfuncno && wfunc->ignore_nulls == perfunc[i].ignore_nulls)

That extra term reads WindowStatePerFuncData.ignore_nulls, but the field
was never populated when a per-function entry was filled in, so it stayed
zero from palloc0_array().  Consequently a duplicate call carrying
IGNORE NULLS or an explicit RESPECT NULLS never matched an identical
earlier entry and was needlessly given its own per-function slot and
evaluated twice.  (Results stayed correct; this was a missed sharing, not
a wrong answer.)

The extra term is in fact redundant.  WindowFunc.ignore_nulls is a plain
scalar field with no pg_node_attr, so _equalWindowFunc() already compares
it; the preceding equal() call therefore never matches two WindowFuncs
that differ only in null treatment.  If equal() matches, ignore_nulls
necessarily matched too, so the term can never change the outcome, and
WindowStatePerFuncData.ignore_nulls existed only to feed it.

Rather than populate the shadow field, drop the redundant term and the
field (and adjust the now-stale comment) and let equal() do the work.
That fixes the same bug while removing the hand-maintained duplicate
state that caused it, so it cannot silently drift again.

Author: Chao Li <li.evan.chao@gmail.com>
Co-authored-by: Ewan Young <kdbase.hack@gmail.com>
Reviewed-by: Tatsuo Ishii <ishii@postgresql.org>
Discussion: https://postgr.es/m/5D2C9081-5DFE-4E27-AB14-7358238EA1BC%40gmail.com
Backpatch-through: 19