Remove LoginFlags from Backend options.

- It was only used in one currently-unused client.
- It's an imperative command, not a configuration setting.
- The LoginFlags stuff in controlclient feels like it needs
  a refactor anyway.

I'll put this logic back once ipnd owns its state and Backend
commands reflect that.

Signed-Off-By: David Anderson <dave@natulte.net>

ipn/local.go

@@ -253,7 +253,7 @@ func (b *LocalBackend) Start(opts Options) error {
 	b.logf("Backend: logs: be:%v fe:%v\n", blid, opts.FrontendLogID)
 	b.send(Notify{BackendLogID: &blid})
 
-	cli.Login(nil, opts.LoginFlags)
+	cli.Login(nil, controlclient.LoginDefault)
 	return nil
 }