Clean up most -Wunused-but-set-variable warnings from gcc 4.6

This warning is new in gcc 4.6 and part of -Wall.  This patch cleans
up most of the noise, but there are some still warnings that are
trickier to remove.

src/backend/rewrite/rewriteDefine.c

@@ -232,7 +232,6 @@ DefineQueryRewrite(char *rulename,
 				   List *action)
 {
 	Relation	event_relation;
-	Oid			ruleId;
 	int			event_attno;
 	ListCell   *l;
 	Query	   *query;
@@ -488,7 +487,7 @@ DefineQueryRewrite(char *rulename,
 	/* discard rule if it's null action and not INSTEAD; it's a no-op */
 	if (action != NIL || is_instead)
 	{
-		ruleId = InsertRule(rulename,
+		InsertRule(rulename,
 							event_type,
 							event_relid,
 							event_attno,