Fix lock behavior on RTCAudioSession.

webrtc/modules/audio_device/ios/objc/RTCAudioSession+Private.h

 /*
  *  Copyright 2016 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.
  */
 
 #import "webrtc/modules/audio_device/ios/objc/RTCAudioSession.h"
 
 NS_ASSUME_NONNULL_BEGIN
 
 @interface RTCAudioSession ()
 
-/** The lock that guards access to AVAudioSession methods. */
-@property(nonatomic, strong) NSRecursiveLock *lock;
-
 /** The delegates. */
 @property(nonatomic, readonly) NSSet *delegates;
 
 /** Number of times setActive:YES has succeeded without a balanced call to
  *  setActive:NO.
  */
 @property(nonatomic, readonly) NSInteger activationCount;
 
 @end
 
 NS_ASSUME_NONNULL_END