Remove unused copy of processed rels in join ordering

The backup list is either a leftover debugging artefact or its
use was removed during the merge work and never made it into the
rewritten commit history. Either way, it serves no purpose so
remove from this hot code path.

Discussion: https://github.com/greenplum-db/gpdb/pull/5216Reviewed-by: NVenkatesh Raghavan <vraghavan@pivotal.io>

src/backend/optimizer/path/allpaths.c

@@ -1444,7 +1444,6 @@ standard_join_search(PlannerInfo *root, int levels_needed, List *initial_rels, b
 		ListCell   *lc;
 		ListCell   *prev;
 		ListCell   *next;
-		List	   *backup;
 
 		/*
 		 * Determine all possible pairs of relations to be joined at this
@@ -1456,7 +1455,6 @@ standard_join_search(PlannerInfo *root, int levels_needed, List *initial_rels, b
 		/*
 		 * Do cleanup work on each just-processed rel.
 		 */
-		backup = list_copy(root->join_rel_level[lev]);
 		prev = NULL;
 		for (lc = list_head(root->join_rel_level[lev]);
 			 lc != NULL;