[auth] Move back to StartOverlay()

StartOverlay2 is now dead code.

SCN-1018 #comment
SCN-1291 #comment

Change-Id: I96c2a3d7f4fbdfe27022203308e12256966bbe18
diff --git a/auth_providers/google/google_auth_provider_impl.cc b/auth_providers/google/google_auth_provider_impl.cc
index 0f4c420..6662950 100644
--- a/auth_providers/google/google_auth_provider_impl.cc
+++ b/auth_providers/google/google_auth_provider_impl.cc
@@ -172,7 +172,7 @@
     return;
   });
 
-  auth_ui_context_->StartOverlay2(std::move(view_holder_token.value));
+  auth_ui_context_->StartOverlay(std::move(view_holder_token));
   ExposeCredentialInjectorInterface();
 }
 
diff --git a/bin/userpicker_base_shell/lib/authentication_ui_context_impl.dart b/bin/userpicker_base_shell/lib/authentication_ui_context_impl.dart
index f806614..1c9ad39 100644
--- a/bin/userpicker_base_shell/lib/authentication_ui_context_impl.dart
+++ b/bin/userpicker_base_shell/lib/authentication_ui_context_impl.dart
@@ -5,7 +5,6 @@
 import 'package:fidl_fuchsia_auth/fidl_async.dart';
 import 'package:fidl_fuchsia_ui_views/fidl_async.dart';
 import 'package:flutter/widgets.dart';
-import 'package:zircon/zircon.dart';
 
 /// Called when an authentication overlay needs to be started.
 typedef OnStartOverlay = void Function(ViewHolderToken viewHolderToken);
@@ -26,15 +25,9 @@
         _onStopOverlay = onStopOverlay;
 
   @override
-  // ignore: override_on_non_overriding_method
   Future<void> startOverlay(ViewHolderToken viewHolderToken) async  =>
       _onStartOverlay?.call(viewHolderToken);
 
   @override
-  // ignore: override_on_non_overriding_method
-  Future<void> startOverlay2(EventPair viewHolderToken) async =>
-      await startOverlay(ViewHolderToken(value: viewHolderToken));
-
-  @override
   Future<void> stopOverlay() async => _onStopOverlay?.call();
 }