Remove base::NonThreadSafe now that all users are gone!!

base/threading/non_thread_safe_impl.cc

-// Copyright (c) 2012 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#include "base/threading/non_thread_safe_impl.h"
-
-#include "base/logging.h"
-
-namespace base {
-
-bool NonThreadSafeImpl::CalledOnValidThread() const {
-  return thread_checker_.CalledOnValidThread();
-}
-
-NonThreadSafeImpl::~NonThreadSafeImpl() {
-  DCHECK(CalledOnValidThread());
-}
-
-void NonThreadSafeImpl::DetachFromThread() {
-  thread_checker_.DetachFromThread();
-}
-
-}  // namespace base