Revert "Force Accessibility Panel to show above all other windows"

This reverts commit 5bb00348.

Reason for revert: Breaks the docked magnifier and breaks the stacking of ash containers.
BUG=969630

Original change's description:
> Force Accessibility Panel to show above all other windows
> 
> Bug: 946616
> Change-Id: I5fa49fe568ab0aecbd2b270c32a714c463d92605
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1542162
> Reviewed-by: David Tseng <dtseng@chromium.org>
> Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
> Cr-Commit-Position: refs/heads/master@{#645374}

TBR=dtseng@chromium.org,anastasi@google.com

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

Bug: 946616
Change-Id: I7ead2ab4cc12499be816fbb2f56ea841ab633242
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1663366Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Auto-Submit: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670167}

ash/accessibility/accessibility_panel_layout_manager.cc

@@ -104,12 +104,6 @@ void AccessibilityPanelLayoutManager::UpdateWindowBounds() {
   RootWindowController* root_controller =
       RootWindowController::ForWindow(root_window);
 
-  aura::Window* current = panel_window_;
-  while (current->parent()) {
-    current->parent()->StackChildAtTop(current);
-    current = current->parent();
-  }
-
   gfx::Rect bounds = panel_bounds_;
 
   // The panel can make itself fill the screen (including covering the shelf).