Obsolete comment in ExecInsert()

Hi,

While reviewing the “Fast COPY FROM based on batch insert" patch, I
noticed this comment introduced in commit b663a4136:

            /*
             * If a certain number of tuples have already been accumulated, or
             * a tuple has come for a different relation than that for the
             * accumulated tuples, perform the batch insert
             */
            if (resultRelInfo->ri_NumSlots == resultRelInfo->ri_BatchSize)
            {
                ExecBatchInsert(mtstate, resultRelInfo,
                                resultRelInfo->ri_Slots,
                                resultRelInfo->ri_PlanSlots,
                                resultRelInfo->ri_NumSlots,
                                estate, canSetTag);
                resultRelInfo->ri_NumSlots = 0;
            }

I think the “or a tuple has come for a different relation than that
for the accumulated tuples" part in the comment is a leftover from an
earlier version of the patch [1].  As the code shows, we do not handle
that case anymore, so I think we should remove that part from the
comment.  Attached is a patch for that.

Best regards,
Etsuro Fujita

[1]
https://www.postgresql.org/message-id/TYAPR01MB2990ECD1C68EA694DD0667E4FEE90%40TYAPR01MB2990.jpnprd01.prod.outlook.com