Resolve a MERGE 12 FIXME in nodeModifyTable.c.
Resolve a MERGE 12 FIXME in nodeModifyTable.c.

Previous bug has already been fixed by 043b809a1e. Now we do support
such kind of SQL:

    WITH updated AS
       (update tbl set rank = 6 where id = 5 returning rank)
      select * from tbl where rank in (select rank from updated);

This commit just removes the comments and change the error message
because this piece of code is not reachable.
xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb