auth: Refactor how authentication methods are passed to server/auth library.

examples/appengine/helloworld_flexible/frontend/main.go

Index: examples/appengine/helloworld_flexible/frontend/main.go
diff --git a/examples/appengine/helloworld_flexible/frontend/main.go b/examples/appengine/helloworld_flexible/frontend/main.go
index 9a37a88f24065e660ce658682cc0d15be491dcc5..b8a1738f33b1f915c25b75dc6f2f47b12852e2a9 100644
--- a/examples/appengine/helloworld_flexible/frontend/main.go
+++ b/examples/appengine/helloworld_flexible/frontend/main.go
@@ -52,15 +52,9 @@ var templateBundle = &templates.Bundle{
 
 // base returns the root middleware chain.
 func base() router.MiddlewareChain {
-	methods := auth.Authenticator{
-		&server.OAuth2Method{Scopes: []string{server.EmailScope}},
-		server.CookieAuth,
-		&server.InboundAppIDAuthMethod{},
-	}
 	return gaemiddleware.BaseProd().Extend(
 		templates.WithTemplates(templateBundle),
-		auth.Use(methods),
-		auth.Authenticate,
+		auth.Authenticate(server.CookieAuth),
 	)
 }