Get rid of adjust_appendrel_attr_needed(), which has been broken ever since

we extended the appendrel mechanism to support UNION ALL optimization.  The
reason nobody noticed was that we are not actually using attr_needed data for
appendrel children; hence it seems more reasonable to rip it out than fix it.
Back-patch to 8.2 because an Assert failure is possible in corner cases.
Per examination of an example from Jim Nasby.

In HEAD, also get rid of AppendRelInfo.col_mappings, which is quite inadequate
to represent UNION ALL situations; depend entirely on translated_vars instead.

src/backend/optimizer/path/allpaths.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.168.2.2 2008/04/01 00:48:44 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.168.2.3 2008/11/11 18:13:43 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -351,14 +351,12 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
 		}
 
 		/*
-		 * Copy the parent's attr_needed data as well, with appropriate
-		 * adjustment of relids and attribute numbers.
+		 * Note: we could compute appropriate attr_needed data for the
+		 * child's variables, by transforming the parent's attr_needed
+		 * through the translated_vars mapping.  However, currently there's
+		 * no need because attr_needed is only examined for base relations
+		 * not otherrels.  So we just leave the child's attr_needed empty.
 		 */
-		pfree(childrel->attr_needed);
-		childrel->attr_needed =
-			adjust_appendrel_attr_needed(rel, appinfo,
-										 childrel->min_attr,
-										 childrel->max_attr);
 
 		/*
 		 * Compute the child's access paths, and add the cheapest one to the