Do not offer the option to be DERP insecure

Websockets, in which DERP is based, requires a TLS certificate. At the same time,
if we use a certificate it must be valid... otherwise Tailscale wont connect (does not
have an Insecure option). So there is no option to expose insecure here

cmd/headscale/cli/utils.go

@@ -118,7 +118,6 @@ func LoadConfig(path string) error {
 
 func GetDERPConfig() headscale.DERPConfig {
 	enabled := viper.GetBool("derp.server.enabled")
-	insecure := viper.GetBool("derp.server.insecure")
 
 	urlStrs := viper.GetStringSlice("derp.urls")
 
@@ -142,7 +141,6 @@ func GetDERPConfig() headscale.DERPConfig {
 
 	return headscale.DERPConfig{
 		ServerEnabled:   enabled,
-		ServerInsecure:  insecure,
 		URLs:            urls,
 		Paths:           paths,
 		AutoUpdate:      autoUpdate,