remove: custom_path_remember() infrastructure

It was reported at heterodb#618.
When # of relations involved in a join exceeds the threshold of GEQO,
PostgreSQL switches optimization logic from the default one.
GEOQ internally uses its own memory context, thus, plan-nodes are
eventually released at end of the GEOQ optimization routine.
It makes impossible to track unused path-nodes, so we changed the
logic to merge joins with multiple-relations.

src/dpu_join.c

@@ -13,7 +13,7 @@
 
 /* static variables */
 static set_join_pathlist_hook_type set_join_pathlist_next = NULL;
-static CustomPathMethods	dpujoin_path_methods;
+       CustomPathMethods	dpujoin_path_methods;
 static CustomScanMethods	dpujoin_plan_methods;
 static CustomExecMethods	dpujoin_exec_methods;
 bool		pgstrom_enable_dpujoin;			/* GUC */

src/dpu_scan.c

@@ -46,15 +46,12 @@ DpuScanAddScanPath(PlannerInfo *root,
 
 		cpath = buildXpuScanPath(root,
 								 baserel,
+								 TASK_KIND__DPUSCAN,
 								 (try_parallel > 0),
 								 true,		/* allow host quals */
 								 false,		/* disallow no device quals */
-								 TASK_KIND__DPUSCAN);
-		if (cpath && custom_path_remember(root,
-										  baserel,
-										  (try_parallel > 0),
-										  TASK_KIND__DPUSCAN,
-										  cpath))
+								 &dpuscan_path_methods);
+		if (cpath)
 		{
 			if (try_parallel == 0)
 				add_path(baserel, &cpath->path);