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.