Correctly set up aggregate FILTER expression in partial-aggregation plans.

The aggfilter expression should be removed from the parent (combining)
Aggref, since it's not supposed to apply the filter, and indeed cannot
because any Vars used in the filter would not be available after the
lower-level aggregation step.  Per report from Jeff Janes.

(This has been broken since the introduction of partial aggregation,
I think.  The error became obvious after commit 59a3795c, when setrefs.c
began processing the parent Aggref's fields normally and thus would detect
such Vars.  The special-case coding previously used in setrefs.c skipped
over the parent's aggfilter field without processing it.  That was broken
in its own way because no other setrefs.c processing got applied either;
though since the executor would not execute the filter expression, only
initialize it, that oversight might not have had any visible symptoms at
present.)

Report: <CAMkU=1xfuPf2edAe4ZGXTmJpU7jxuKukKyvNtEXwu35B7dvejg@mail.gmail.com>