Change from custom Lock to std::mutex.

Rest of merge of http://go/wvgerrit/67884
Some files were deleted on original CL that were not deleted here.

Now that we can use C++11, we should use the cross-platform std::mutex
type, not the custom pthread version.

Bug: 111850982
Test: WV unit/integration tests
Change-Id: I48a1e47aa79e5e66b5869c0f766c18d561d26784

libwvdrmengine/cdm/util/src/lock.cpp

@@ -1,32 +0,0 @@
-// Copyright 2018 Google LLC. All Rights Reserved. This file and proprietary
-// source code may only be used and distributed under the Widevine Master
-// License Agreement.
-//
-// Lock class - provides a simple android specific mutex implementation
-
-#include "lock.h"
-#include <utils/Mutex.h>
-
-namespace wvcdm {
-
-class Lock::Impl {
- public:
-  android::Mutex lock_;
-};
-
-Lock::Lock() : impl_(new Lock::Impl()) {
-}
-
-Lock::~Lock() {
-  delete impl_;
-}
-
-void Lock::Acquire() {
-  impl_->lock_.lock();
-}
-
-void Lock::Release() {
-  impl_->lock_.unlock();
-}
-
-}  // namespace wvcdm