Revert "Call preserveSurfaces when detaching children."

This reverts commit 9118c9b573.

Reason for revert: b/161316231

Bug: 161316231
Test: Steps from bug
Change-Id: Ieb76c99d96eb7f261a1bd2680ebf83905c2a5369

services/core/java/com/android/server/wm/WindowStateAnimator.java

@@ -1592,12 +1592,6 @@ class WindowStateAnimator {
         if (mSurfaceController != null) {
             mSurfaceController.detachChildren();
         }
-        // If the children are detached, it means the app is exiting. We don't want to tear the
-        // content down too early, otherwise we could end up with a flicker. By preserving the
-        // current surface, we ensure the content remains on screen until the window is completely
-        // removed. It also ensures that the old surface is cleaned up when started again since it
-        // forces mSurfaceController to be set to null.
-        preserveSurfaceLocked();
     }
 
     void setOffsetPositionForStackResize(boolean offsetPositionForStackResize) {