math,net: omit explicit true tag expr in switch

Performed `switch true {}` => `switch {}` replacement.

Found using https://go-critic.github.io/overview.html#switchTrue-ref

Change-Id: Ib39ea98531651966a5a56b7bd729b46e4eeb7f7c
Reviewed-on: https://go-review.googlesource.com/123378
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/math/sinh.go

@@ -43,7 +43,7 @@ func sinh(x float64) float64 {
 	}
 
 	var temp float64
-	switch true {
+	switch {
 	case x > 21:
 		temp = Exp(x) * 0.5
 

src/net/ip.go

@@ -222,7 +222,7 @@ func (ip IP) DefaultMask() IPMask {
 	if ip = ip.To4(); ip == nil {
 		return nil
 	}
-	switch true {
+	switch {
 	case ip[0] < 0x80:
 		return classAMask
 	case ip[0] < 0xC0: