math/big: Remove unnecessary test from nat.go multiplication

The switch at the beginning of the function already ensures n > 1,
so testing for n < 2 is redundant.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/6350051

src/pkg/math/big/nat.go

@@ -396,7 +396,7 @@ func (z nat) mul(x, y nat) nat {
 	}
 
 	// use basic multiplication if the numbers are small
-	if n < karatsubaThreshold || n < 2 {
+	if n < karatsubaThreshold {
 		z = z.make(m + n)
 		basicMul(z, x, y)
 		return z.norm()