Remove last vestigial uses of "update needed" png assets.

chrome/app/theme/theme_resources.grd

Index: chrome/app/theme/theme_resources.grd
diff --git a/chrome/app/theme/theme_resources.grd b/chrome/app/theme/theme_resources.grd
index 868e023a01f231013dac6c0dac364cb5536b4c08..7f3a79aa5648ca181b137753afe3f5ce8739d86d 100644
--- a/chrome/app/theme/theme_resources.grd
+++ b/chrome/app/theme/theme_resources.grd
@@ -658,9 +658,6 @@
         <structure type="chrome_scaled_image" name="IDR_TRANSLATE" file="legacy/translate.png" />
         <structure type="chrome_scaled_image" name="IDR_TRANSLATE_ACTIVE" file="legacy/translate_active.png" />
       </if>
-      <structure type="chrome_scaled_image" name="IDR_UPDATE_MENU_SEVERITY_LOW" file="common/update_menu_severity_low.png" />
-      <structure type="chrome_scaled_image" name="IDR_UPDATE_MENU_SEVERITY_MEDIUM" file="common/update_menu_severity_medium.png" />
-      <structure type="chrome_scaled_image" name="IDR_UPDATE_MENU_SEVERITY_HIGH" file="common/update_menu_severity_high.png" />
       <structure type="chrome_scaled_image" name="IDR_USB_NOTIFICATION_ICON" file="common/notification_usb_icon.png" />
       <if expr="chromeos">
         <structure type="chrome_scaled_image" name="IDR_USER_IMAGE_CAPTURE" file="cros/snapshot_wide.png" />