Revert "Enable optimizer_use_gpdb_allocators guc by default (#8771)"

This reverts commit b81d4501.

The resource group tests use ORCA and need to be modified for this
change.

src/backend/utils/misc/guc_gp.c

@@ -2799,7 +2799,7 @@ struct config_bool ConfigureNamesBool_gp[] =
 			GUC_NO_SHOW_ALL | GUC_NOT_IN_SAMPLE
 		},
 		&optimizer_use_gpdb_allocators,
-		true,
+		false,
 		NULL, NULL, NULL
 	},