Remove unused panel frame assets.

ui/resources/ui_resources.grd

Index: ui/resources/ui_resources.grd
diff --git a/ui/resources/ui_resources.grd b/ui/resources/ui_resources.grd
index 7dd3885c9fde55a9d9f0a58344b724afa7e0b200..ea73b3667bc108525fb524d4ad29c877f3c05642 100644
--- a/ui/resources/ui_resources.grd
+++ b/ui/resources/ui_resources.grd
@@ -174,12 +174,6 @@
         <structure type="chrome_scaled_image" name="IDR_OOBE_ACTION_BOX_BUTTON_NORMAL" file="cros/action_box_button_normal.png" />
         <structure type="chrome_scaled_image" name="IDR_OOBE_ACTION_BOX_BUTTON_PRESSED" file="cros/action_box_button_pressed.png" />
       </if>
-      <if expr="toolkit_views">
-        <structure type="chrome_scaled_image" name="IDR_PANEL_TOP_LEFT_CORNER" file="panel_top_left_corner.png" />
-        <structure type="chrome_scaled_image" name="IDR_PANEL_TOP_RIGHT_CORNER" file="panel_top_right_corner.png" />
-        <structure type="chrome_scaled_image" name="IDR_PANEL_BOTTOM_LEFT_CORNER" file="panel_bottom_left_corner.png" />
-        <structure type="chrome_scaled_image" name="IDR_PANEL_BOTTOM_RIGHT_CORNER" file="panel_bottom_right_corner.png" />
-      </if>
       <if expr="not is_android and not is_ios">
         <structure type="chrome_scaled_image" name="IDR_SIGNAL_0_BAR" file="common/signal_0_bar.png" />
         <structure type="chrome_scaled_image" name="IDR_SIGNAL_1_BAR" file="common/signal_1_bar.png" />