Cosmetic improvements in setup of planner's per-RTE arrays.

Merge setup_append_rel_array into setup_simple_rel_arrays.  There's no
particularly good reason to keep them separate, and it's inconsistent
with the lack of separation in expand_planner_arrays.  The only apparent
benefit was that the fast path for trivial queries in query_planner()
doesn't need to set up the append_rel_array; but all we're saving there
is an if-test and NULL assignment, which surely ought to be negligible.

Also improve some obsolete comments.

Discussion: https://postgr.es/m/17220.1565301350@sss.pgh.pa.us

src/backend/optimizer/plan/planmain.c

@@ -79,9 +79,7 @@ query_planner(PlannerInfo *root,
 	root->initial_rels = NIL;
 
 	/*
-	 * Make a flattened version of the rangetable for faster access (this is
-	 * OK because the rangetable won't change any more), and set up an empty
-	 * array for indexing base relations.
+	 * Set up arrays for accessing base relations and AppendRelInfos.
 	 */
 	setup_simple_rel_arrays(root);
 
@@ -156,12 +154,6 @@ query_planner(PlannerInfo *root,
 		}
 	}
 
-	/*
-	 * Populate append_rel_array with each AppendRelInfo to allow direct
-	 * lookups by child relid.
-	 */
-	setup_append_rel_array(root);
-
 	/*
 	 * Construct RelOptInfo nodes for all base relations used in the query.
 	 * Appendrel member relations ("other rels") will be added later.