DEV: Remove the old header widgets code (#28390)

Remove the header widget code.

More info can be found in https://meta.discourse.org/t/upcoming-header-changes-preparing-themes-and-plugins/296544

db/migrate/20240815234500_remove_glimmer_header_mode_setting.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+#
+class RemoveGlimmerHeaderModeSetting < ActiveRecord::Migration[7.1]
+  def up
+    execute <<~SQL
+      DELETE FROM site_settings
+      WHERE name = 'glimmer_header_mode'
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end