wgengine/router: don't double-prefix dns log messages [Windows]

(cherry picked from commit 119101962c)

wgengine/router/dns/config.go

@@ -63,7 +63,8 @@ func (lhs Config) Equal(rhs Config) bool {
 // ManagerConfig is the set of parameters from which
 // a manager implementation is chosen and initialized.
 type ManagerConfig struct {
-	// logf is the logger for the manager to use.
+	// Logf is the logger for the manager to use.
+	// It is wrapped with a "dns: " prefix.
 	Logf logger.Logf
 	// InterfaceNAme is the name of the interface with which DNS settings should be associated.
 	InterfaceName string

wgengine/router/router_windows.go

@@ -39,7 +39,7 @@ func newUserspaceRouter(logf logger.Logf, wgdev *device.Device, tundev tun.Devic
 	nativeTun := tundev.(*tun.NativeTun)
 	guid := nativeTun.GUID().String()
 	mconfig := dns.ManagerConfig{
-		Logf:          logger.WithPrefix(logf, "dns: "),
+		Logf:          logf,
 		InterfaceName: guid,
 	}