UX: Enable new notifications menu by default (#21060)

https://meta.discourse.org/t/260358

lib/validators/enable_new_notifications_menu_validator.rb

@@ -1,15 +0,0 @@
-# frozen_string_literal: true
-
-class EnableNewNotificationsMenuValidator
-  def initialize(opts = {})
-  end
-
-  def valid_value?(value)
-    return true if value == "f"
-    SiteSetting.navigation_menu == "legacy"
-  end
-
-  def error_message
-    I18n.t("site_settings.errors.enable_new_notifications_menu_not_legacy_navigation_menu")
-  end
-end