Remove TupleDesc remapping logic from tqueue.c.

With the introduction of a shared memory record typmod registry, it is no
longer necessary to remap record typmods when sending tuples between backends
so most of tqueue.c can be removed.

Author: Thomas Munro
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/CAEepm=0ZtQ-SpsgCyzzYpsXS6e=kZWqk3g5Ygn3MDV7A8dabUA@mail.gmail.com

src/backend/executor/execParallel.c

@@ -608,14 +608,12 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate, int nworkers,
 
 /*
  * Set up tuple queue readers to read the results of a parallel subplan.
- * All the workers are expected to return tuples matching tupDesc.
  *
  * This is separate from ExecInitParallelPlan() because we can launch the
  * worker processes and let them start doing something before we do this.
  */
 void
-ExecParallelCreateReaders(ParallelExecutorInfo *pei,
-						  TupleDesc tupDesc)
+ExecParallelCreateReaders(ParallelExecutorInfo *pei)
 {
 	int			nworkers = pei->pcxt->nworkers_launched;
 	int			i;
@@ -631,8 +629,7 @@ ExecParallelCreateReaders(ParallelExecutorInfo *pei,
 		{
 			shm_mq_set_handle(pei->tqueue[i],
 							  pei->pcxt->worker[i].bgwhandle);
-			pei->reader[i] = CreateTupleQueueReader(pei->tqueue[i],
-													tupDesc);
+			pei->reader[i] = CreateTupleQueueReader(pei->tqueue[i]);
 		}
 	}
 }

src/backend/executor/nodeGather.c

@@ -176,8 +176,7 @@ ExecGather(PlanState *pstate)
 			/* Set up tuple queue readers to read the results. */
 			if (pcxt->nworkers_launched > 0)
 			{
-				ExecParallelCreateReaders(node->pei,
-										  fslot->tts_tupleDescriptor);
+				ExecParallelCreateReaders(node->pei);
 				/* Make a working array showing the active readers */
 				node->nreaders = pcxt->nworkers_launched;
 				node->reader = (TupleQueueReader **)

src/backend/executor/nodeGatherMerge.c

@@ -217,7 +217,7 @@ ExecGatherMerge(PlanState *pstate)
 			/* Set up tuple queue readers to read the results. */
 			if (pcxt->nworkers_launched > 0)
 			{
-				ExecParallelCreateReaders(node->pei, node->tupDesc);
+				ExecParallelCreateReaders(node->pei);
 				/* Make a working array showing the active readers */
 				node->nreaders = pcxt->nworkers_launched;
 				node->reader = (TupleQueueReader **)