Fixed ineffectual assignments (#7555)

Don't assign values we never use.

routers/user/setting/security_twofa.go

@@ -81,7 +81,6 @@ func twofaGenerateSecretAndQr(ctx *context.Context) bool {
 	// Filter unsafe character ':' in issuer
 	issuer := strings.Replace(setting.AppName+" ("+setting.Domain+")", ":", "", -1)
 	if otpKey == nil {
-		err = nil // clear the error, in case the URL was invalid
 		otpKey, err = totp.Generate(totp.GenerateOpts{
 			SecretSize:  40,
 			Issuer:      issuer,