pgsql: Fix out-dated comment in preprocess_groupclause()

Fix out-dated comment in preprocess_groupclause()

The comment claimed we don't consider other orders of the GROUP BY clause,
but this is no longer true as of db0d67db2.

Discussion: https://postgr.es/m/CAApHDvq65=9Ro+hLX1i9ugWEiNDvHrBibAO7ARcTnf38_JE+UQ@mail.gmail.com
Backpatch-through: 15, where db0d67db2 was introduced.

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f6c70b81802a869303be9916d1a64a15f4dd29d1

Modified Files
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src/backend/optimizer/plan/planner.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)