pgsql: Don't pass the grouping target around unnecessarily.

Don't pass the grouping target around unnecessarily.

Since commit 4f15e5d09de276fb77326be5567dd9796008ca2e made grouped_rel
set reltarget, a variety of other functions can just get it from
grouped_rel instead of having to pass it around explicitly.  Simplify
accordingly.

Patch by me, reviewed by Ashutosh Bapat.

Discussion: http://postgr.es/m/CA+TgmoZ+ZJTVad-=vEq393N99KTooxv9k7M+z73qnTAqkb49BQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/94150513ec12c13eb7c98430fc34f477896d38c9

Modified Files
--------------
src/backend/optimizer/plan/planner.c  | 41 +++++++++++++----------------------
src/backend/optimizer/util/pathnode.c |  4 ++--
src/include/optimizer/pathnode.h      |  2 --
3 files changed, 17 insertions(+), 30 deletions(-)