Revert "Enable parallel SELECT for "INSERT INTO ... SELECT ..."."

To allow inserts in parallel-mode this feature has to ensure that all the
constraints, triggers, etc. are parallel-safe for the partition hierarchy
which is costly and we need to find a better way to do that. Additionally,
we could have used existing cached information in some cases like indexes,
domains, etc. to determine the parallel-safety.

List of commits reverted, in reverse chronological order:

ed62d3737c Doc: Update description for parallel insert reloption.
c8f78b6161 Add a new GUC and a reloption to enable inserts in parallel-mode.
c5be48f092 Improve FK trigger parallel-safety check added by 05c8482f7f.
e2cda3c20a Fix use of relcache TriggerDesc field introduced by commit 05c8482f7f.
e4e87a32cc Fix valgrind issue in commit 05c8482f7f.
05c8482f7f Enable parallel SELECT for "INSERT INTO ... SELECT ...".

Discussion: https://postgr.es/m/E1lMiB9-0001c3-SY@gemulon.postgresql.org

src/backend/access/transam/xact.c

@@ -1014,32 +1014,6 @@ IsInParallelMode(void)
 	return CurrentTransactionState->parallelModeLevel != 0;
 }
 
-/*
- *	PrepareParallelModePlanExec
- *
- * Prepare for entering parallel mode plan execution, based on command-type.
- */
-void
-PrepareParallelModePlanExec(CmdType commandType)
-{
-	if (IsModifySupportedInParallelMode(commandType))
-	{
-		Assert(!IsInParallelMode());
-
-		/*
-		 * Prepare for entering parallel mode by assigning a TransactionId.
-		 * Failure to do this now would result in heap_insert() subsequently
-		 * attempting to assign a TransactionId whilst in parallel-mode, which
-		 * is not allowed.
-		 *
-		 * This approach has a disadvantage in that if the underlying SELECT
-		 * does not return any rows, then the TransactionId is not used,
-		 * however that shouldn't happen in practice in many cases.
-		 */
-		(void) GetCurrentTransactionId();
-	}
-}
-
 /*
  *	CommandCounterIncrement
  */