[Sync] Remove unneeded 'using syncer::' lines and 'syncer::' scopings

sync/test/null_transaction_observer.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 "sync/test/null_transaction_observer.h"
 
 #include "base/memory/weak_ptr.h"
 
 namespace syncer {
 namespace syncable {
 
-syncer::WeakHandle<TransactionObserver> NullTransactionObserver() {
-  return syncer::MakeWeakHandle(base::WeakPtr<TransactionObserver>());
+WeakHandle<TransactionObserver> NullTransactionObserver() {
+  return MakeWeakHandle(base::WeakPtr<TransactionObserver>());
 }
 
 }  // namespace syncable
 }  // namespace syncer