Move SystemInfo to rtc_base_approved and delete unused code.

webrtc/system_wrappers/interface/cpu_info.h

 /*
  *  Copyright (c) 2011 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #ifndef WEBRTC_SYSTEM_WRAPPERS_INTERFACE_CPU_INFO_H_
 #define WEBRTC_SYSTEM_WRAPPERS_INTERFACE_CPU_INFO_H_
 
 #include "webrtc/typedefs.h"
 
 namespace webrtc {
 
 class CpuInfo {
  public:
   static uint32_t DetectNumberOfCores();
 
  private:
   CpuInfo() {}
-  static uint32_t number_of_cores_;
 };
 
 }  // namespace webrtc
 
 #endif // WEBRTC_SYSTEM_WRAPPERS_INTERFACE_CPU_INFO_H_