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/backend.go

@@ -5,10 +5,11 @@
 package ipn
 
 import (
+	"time"
+
 	"tailscale.com/control/controlclient"
 	"tailscale.com/tailcfg"
 	"tailscale.com/wgengine"
-	"time"
 )
 
 type State int
@@ -53,7 +54,6 @@ type Options struct {
 	FrontendLogID string // public logtail id used by frontend
 	ServerURL     string
 	Prefs         Prefs
-	LoginFlags    controlclient.LoginFlags
 	Notify        func(n Notify) `json:"-"`
 }