throughout: simplify two-variable ranges with unused second variable

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/3529041
diff --git a/src/pkg/unicode/maketables.go b/src/pkg/unicode/maketables.go
index c8e7eb4..081e1a1 100644
--- a/src/pkg/unicode/maketables.go
+++ b/src/pkg/unicode/maketables.go
@@ -326,7 +326,7 @@
 	if *tablelist == "all" {
 		fmt.Println("// Categories is the set of Unicode data tables.")
 		fmt.Println("var Categories = map[string] []Range {")
-		for k, _ := range category {
+		for k := range category {
 			fmt.Printf("\t%q: %s,\n", k, k)
 		}
 		fmt.Print("}\n\n")
@@ -594,7 +594,7 @@
 			fmt.Println("// Scripts is the set of Unicode script tables.")
 			fmt.Println("var Scripts = map[string] []Range {")
 		}
-		for k, _ := range table {
+		for k := range table {
 			fmt.Printf("\t%q: %s,\n", k, k)
 		}
 		fmt.Print("}\n\n")