Improve make_subplanTargetList to avoid including Vars unnecessarily.

If a Var was used only in a GROUP BY expression, the previous
implementation would include the Var by itself (as well as the expression)
in the generated targetlist.  This wouldn't affect the efficiency of the
scan/join part of the plan at all, but it could result in passing
unnecessarily-wide rows through sorting and grouping steps.  It turns out
to take only a little more code, and not noticeably more time, to generate
a tlist without such redundancy, so let's do that.  Per a recent gripe from
HarmeekSingh Bedi.