Revert "cros: Fix ml::UserActivityManager shutdown crash"

This reverts commit 2c8bdfae.

Reason for revert:
https://crrev.com/c/1719447 makes the CL unnecessary.

Original change's description:
> cros: Fix ml::UserActivityManager shutdown crash
> 
> SessionManager is released in PostMainMessageLoopRun that
> happens before PostDestroyThreads. ml::UserActivityManager
> is released with DbusServices PostDestroyThreads. Hence it
> crashes on shutdown when trying to un-observe SessionManager.
> 
> This CL adds a OnSessionManagerDestroyed to SessionManagerObserver
> interface so that ml::UserActivityManager could clean up in time.
> 
> Bug: 987010
> Change-Id: I05043a3e0b946bfb9e733ad476dacc35e9a9a3b6
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715401
> Reviewed-by: Jia Meng <jiameng@chromium.org>
> Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#680197}

TBR=xiyuan@chromium.org,jiameng@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 987010
Change-Id: I7fbd5cc8d3e693c7d6b1ceb169d10914040afa17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1720999Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681436}

chrome/browser/chromeos/power/ml/user_activity_manager.cc

@@ -306,11 +306,6 @@ void UserActivityManager::HandleSmartDimDecision(
   std::move(callback).Run(dim_deferred_);
 }
 
-void UserActivityManager::OnSessionManagerDestroyed() {
-  session_manager_observer_.RemoveAll();
-  session_manager_ = nullptr;
-}
-
 void UserActivityManager::OnSessionStateChanged() {
   DCHECK(session_manager_);
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);

chrome/browser/chromeos/power/ml/user_activity_manager.h

@@ -117,7 +117,6 @@ class UserActivityManager : public ui::UserActivityObserver,
                               UserActivityEvent::ModelPrediction prediction);
 
   // session_manager::SessionManagerObserver overrides:
-  void OnSessionManagerDestroyed() override;
   void OnSessionStateChanged() override;
 
  private:
@@ -203,7 +202,7 @@ class UserActivityManager : public ui::UserActivityObserver,
                  session_manager::SessionManagerObserver>
       session_manager_observer_;
 
-  session_manager::SessionManager* session_manager_ = nullptr;
+  session_manager::SessionManager* const session_manager_;
 
   mojo::Binding<viz::mojom::VideoDetectorObserver> binding_;
 

components/session_manager/core/session_manager.cc

@@ -24,9 +24,6 @@ SessionManager::SessionManager() {
 SessionManager::~SessionManager() {
   DCHECK_EQ(instance, this);
   SessionManager::SetInstance(nullptr);
-
-  for (auto& observer : observers_)
-    observer.OnSessionManagerDestroyed();
 }
 
 // static

components/session_manager/core/session_manager_observer.h

@@ -16,9 +16,6 @@ namespace session_manager {
 //     http://crbug.com/657149.
 class SessionManagerObserver : public base::CheckedObserver {
  public:
-  // Inovked when session manager is destroyed.
-  virtual void OnSessionManagerDestroyed() {}
-
   // Invoked when session state is changed.
   virtual void OnSessionStateChanged() {}