overview: Remove unnecessary code in OverviewItem::PrepareForOverview

The present CL removes some code in OverviewItem::PrepareForOverview
because the same code is in OverviewItem::CreateItemWidget and is not
needed twice.

Bug: None
Change-Id: I3d9acec30eb96a645faece519f47352fb3302a01
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1993708Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730015}

ash/wm/overview/overview_item.cc

@@ -251,9 +251,6 @@ void OverviewItem::Shutdown() {
 
 void OverviewItem::PrepareForOverview() {
   transform_window_.PrepareForOverview();
-  aura::Window* widget_window = item_widget_->GetNativeWindow();
-  widget_window->parent()->StackChildBelow(widget_window, GetWindow());
-
   prepared_for_overview_ = true;
 }