math: Remove redundant local variable Ln2

Use the const variable Ln2 in math/const.go for function acosh.

Change-Id: I5381d03dd3142c227ae5773ece9be6c8f377615e
Reviewed-on: https://go-review.googlesource.com/c/go/+/232517
Reviewed-by: Robert Griesemer <[email protected]>
Trust: Robert Griesemer <[email protected]>
Trust: Giovanni Bajo <[email protected]>

Author: surechen

src/math/acosh.go

@@ -42,10 +42,7 @@ package math
 func Acosh(x float64) float64
 
 func acosh(x float64) float64 {
-	const (
-		Ln2   = 6.93147180559945286227e-01 // 0x3FE62E42FEFA39EF
-		Large = 1 << 28                    // 2**28
-	)
+	const Large = 1 << 28 // 2**28
 	// first case is special case
 	switch {
 	case x < 1 || IsNaN(x):