Merge pull request #26907 from y-yagi/remove_unused_RedirectBackError

remove unused `RedirectBackError` class

actionpack/lib/action_controller/metal/redirecting.rb

 module ActionController
-  class RedirectBackError < AbstractController::Error #:nodoc:
-    DEFAULT_MESSAGE = 'No HTTP_REFERER was set in the request to this action, so redirect_to :back could not be called successfully. If this is a test, make sure to specify request.env["HTTP_REFERER"].'
-
-    def initialize(message = nil)
-      super(message || DEFAULT_MESSAGE)
-    end
-  end
-
   module Redirecting
     extend ActiveSupport::Concern