SurfaceFlinger: remove mUse90Hz flag

As we have now DisplayManager changes in place (b/122905403)
there is no need for mUse90Hz flag. 90Hz will be enabled by
DisplayManager.

Fixes: 126725468
Test: Enable peak refresh from settings
Change-Id: I949727f8ba41bba874ab07160f3f6153d3bf4c67
diff --git a/services/surfaceflinger/Scheduler/Scheduler.h b/services/surfaceflinger/Scheduler/Scheduler.h
index f8667d0..5b9759b 100644
--- a/services/surfaceflinger/Scheduler/Scheduler.h
+++ b/services/surfaceflinger/Scheduler/Scheduler.h
@@ -148,6 +148,9 @@
     void incrementFrameCounter();
     // Callback that gets invoked when Scheduler wants to change the refresh rate.
     void setChangeRefreshRateCallback(const ChangeRefreshRateCallback& changeRefreshRateCallback);
+
+    // Returns whether idle timer is enabled or not
+    bool isIdleTimerEnabled() { return mSetIdleTimerMs > 0; }
     // Returns relevant information about Scheduler for dumpsys purposes.
     std::string doDump();