pgsql: Fix test case from 40c242830

Fix test case from 40c242830

I mistakenly included the "Replaces" lines describing the origin of
Result nodes in groupingsets.out, which actually come from a feature
not available in v18.  Mea culpa.

Per buildfarm.

Discussion: https://postgr.es/m/CAMbWs4_VxjdM-nBvt8YE=84rE4OLBES27Wz1P0=9Z6KgwPqzEA@mail.gmail.com

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/ee49f2cf447ab3191164aec82410dc52f7960c6f

Modified Files
--------------
src/test/regress/expected/groupingsets.out | 21 +++++++++------------
1 file changed, 9 insertions(+), 12 deletions(-)