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/index/suffixarray/suffixarray.go b/src/pkg/index/suffixarray/suffixarray.go
index 4839dbb..2d728e2 100644
--- a/src/pkg/index/suffixarray/suffixarray.go
+++ b/src/pkg/index/suffixarray/suffixarray.go
@@ -40,7 +40,7 @@
 //
 func New(data []byte) *Index {
 	sa := make([]int, len(data))
-	for i, _ := range sa {
+	for i := range sa {
 		sa[i] = i
 	}
 	x := &Index{data, sa}