chromeos: Clean up ash feature flag UseLoginShelfWidget

The flag places the login shelf view in its own widget instead of
sharing the shelf widget. The flag has been on-by-default since July
2022. There are no references in chromeos (e.g. Tast) or in google3
(e.g. experiment configs). Delete the flag and clean up the code.

Bug: none
Change-Id: I67206763b3c0c75581ec15e5f60d939af2a46368
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5271021
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1256868}

diff --git a/ash/login/login_screen_controller.cc b/ash/login/login_screen_controller.cc
index ee062bc..d26d429 100644
--- a/ash/login/login_screen_controller.cc
+++ b/ash/login/login_screen_controller.cc

@@ -326,14 +326,9 @@
     Shell::Get()->focus_cycler()->FocusWidget(shelf->GetStatusAreaWidget());
   } else if (shelf->shelf_widget()->GetLoginShelfView()->IsFocusable()) {
     // Otherwise focus goes to login shelf buttons when there is any.
-    if (features::IsUseLoginShelfWidgetEnabled()) {
-      LoginShelfWidget* login_shelf_widget = shelf->login_shelf_widget();
-      login_shelf_widget->SetDefaultLastFocusableChild(reverse);
-      Shell::Get()->focus_cycler()->FocusWidget(login_shelf_widget);
-    } else {
-      shelf->shelf_widget()->set_default_last_focusable_child(reverse);
-      Shell::Get()->focus_cycler()->FocusWidget(shelf->shelf_widget());
-    }
+    LoginShelfWidget* login_shelf_widget = shelf->login_shelf_widget();
+    login_shelf_widget->SetDefaultLastFocusableChild(reverse);
+    Shell::Get()->focus_cycler()->FocusWidget(login_shelf_widget);
   } else {
     // No elements to focus on the shelf.
     //