nixos/headscale: remove much-loosened-up server_url check (#374374)

  config = lib.mkIf cfg.enable {

    assertions = [

      {

        # This is stricter than it needs to be but is exactly what upstream does:

        # https://github.com/kradalby/headscale/blob/adc084f20f843d7963c999764fa83939668d2d2c/hscontrol/types/config.go#L799

        assertion =

          with cfg.settings;

          dns.use_username_in_magic_dns or false

          || dns.base_domain == ""

          || !lib.hasInfix dns.base_domain server_url;

        message = "server_url cannot contain the base_domain, this will cause the headscale server and embedded DERP to become unreachable from the Tailscale node.";

      }

      {

        assertion = with cfg.settings; dns.magic_dns -> dns.base_domain != "";

        message = "dns.base_domain must be set when using MagicDNS";