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

config-example.yaml

@@ -57,12 +57,9 @@ ip_prefixes:
 derp:
   server:
     # If enabled, runs the embedded DERP server and merges it into the rest of the DERP config
+    # The Headscale server_url defined above MUST be using https, DERP requires TLS to be in place
     enabled: false
 
-    # Insecure mode is recommended only for tests. It indicates the tailscale clients
-    # to use insecure connections to this server.
-    insecure: false
-
   # List of externally available DERP maps encoded in JSON
   urls:
     - https://controlplane.tailscale.com/derpmap/default