Temporarily revert test case from a2b70c89ca1a5fcf6181d3c777d82e7b83d2de1b.

That code patch was good as far as it went, but the associated test case
has exposed fundamental brain damage in the parallel scan mechanism,
which is going to take nontrivial work to correct.  In the interests of
getting the buildfarm back to green so that unrelated work can proceed,
let's temporarily remove the test case.

src/test/regress/expected/select_parallel.out

@@ -300,49 +300,6 @@ select count(*) from tenk1 group by twenty;
    500
 (20 rows)
 
---test rescan behavior of gather merge
-set enable_material = false;
-explain (costs off)
-select * from
-  (select string4, count(unique2)
-   from tenk1 group by string4 order by string4) ss
-  right join (values (1),(2),(3)) v(x) on true;
-                        QUERY PLAN                        
-----------------------------------------------------------
- Nested Loop Left Join
-   ->  Values Scan on "*VALUES*"
-   ->  Finalize GroupAggregate
-         Group Key: tenk1.string4
-         ->  Gather Merge
-               Workers Planned: 4
-               ->  Partial GroupAggregate
-                     Group Key: tenk1.string4
-                     ->  Sort
-                           Sort Key: tenk1.string4
-                           ->  Parallel Seq Scan on tenk1
-(11 rows)
-
-select * from
-  (select string4, count(unique2)
-   from tenk1 group by string4 order by string4) ss
-  right join (values (1),(2),(3)) v(x) on true;
- string4 | count | x 
----------+-------+---
- AAAAxx  |  2500 | 1
- HHHHxx  |  2500 | 1
- OOOOxx  |  2500 | 1
- VVVVxx  |  2500 | 1
- AAAAxx  |  2500 | 2
- HHHHxx  |  2500 | 2
- OOOOxx  |  2500 | 2
- VVVVxx  |  2500 | 2
- AAAAxx  |  2500 | 3
- HHHHxx  |  2500 | 3
- OOOOxx  |  2500 | 3
- VVVVxx  |  2500 | 3
-(12 rows)
-
-reset enable_material;
 -- gather merge test with 0 worker
 set max_parallel_workers = 0;
 explain (costs off)

src/test/regress/sql/select_parallel.sql

@@ -118,22 +118,6 @@ explain (costs off)
 
 select count(*) from tenk1 group by twenty;
 
---test rescan behavior of gather merge
-set enable_material = false;
-
-explain (costs off)
-select * from
-  (select string4, count(unique2)
-   from tenk1 group by string4 order by string4) ss
-  right join (values (1),(2),(3)) v(x) on true;
-
-select * from
-  (select string4, count(unique2)
-   from tenk1 group by string4 order by string4) ss
-  right join (values (1),(2),(3)) v(x) on true;
-
-reset enable_material;
-
 -- gather merge test with 0 worker
 set max_parallel_workers = 0;
 explain (costs off)