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/e2e_test.go

@@ -177,7 +177,6 @@ func newNode(t *testing.T, prefix string, https *httptest.Server) testNode {
 			WantRunning:      true,
 			Persist:          &c,
 		},
-		LoginFlags: controlclient.LoginDefault,
 		Notify: func(n Notify) {
 			// Automatically visit auth URLs
 			if n.BrowseToURL != nil {