Revert "cmd/tailscaled, ipn/{ipnlocal,ipnserver}: let netstack get access to LocalBackend"

This reverts commit 2bc518dcb2.

@namansood didn't end up needing it in his 770aa71ffb.

ipn/ipnlocal/future.go

@@ -1,28 +0,0 @@
-// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package ipnlocal
-
-import "sync"
-
-// LocalBackendFuture is a Future that returns a *LocalBackend.
-type LocalBackendFuture struct {
-	getOnce sync.Once
-	ch      chan *LocalBackend
-	v       *LocalBackend
-}
-
-func (f *LocalBackendFuture) Get() *LocalBackend {
-	f.getOnce.Do(f.get)
-	return f.v
-}
-
-func (f *LocalBackendFuture) get()                { f.v = <-f.ch }
-func (f *LocalBackendFuture) Set(v *LocalBackend) { f.ch <- v }
-
-func NewLocalBackendFuture() *LocalBackendFuture {
-	return &LocalBackendFuture{
-		ch: make(chan *LocalBackend, 1),
-	}
-}