math: avoid unused assignment in jn.go

Change-Id: Ie4f21bcd5849e994c63ec5bbda2dee6f3ec4da12
Reviewed-on: https://go-review.googlesource.com/13891Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/math/jn.go

@@ -200,13 +200,11 @@ func Jn(n int, x float64) float64 {
 				for i := n - 1; i > 0; i-- {
 					di := float64(i + i)
 					a, b = b, b*di/x-a
-					di -= 2
 				}
 			} else {
 				for i := n - 1; i > 0; i-- {
 					di := float64(i + i)
 					a, b = b, b*di/x-a
-					di -= 2
 					// scale b to avoid spurious overflow
 					if b > 1e100 {
 						a /= b