Remove unused function from update_screen.h

This CL removes GetErrorScreen(). This function is not implemented and
has no callers.

Change-Id: Icffc713e6534c14f1f2feb6cb030e7f4b57e561e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1598268Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Tony De Luna <tonydeluna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657334}

chrome/browser/chromeos/login/screens/update_screen.h

@@ -148,9 +148,6 @@ class UpdateScreen : public BaseScreen,
   // Checks that screen is shown, shows if not.
   void MakeSureScreenIsShown();
 
-  // Returns an instance of the error screen.
-  ErrorScreen* GetErrorScreen();
-
   void StartUpdateCheck();
   void ShowErrorMessage();
   void HideErrorMessage();