Replace duplicate method with module inclusion

No reason to risk diverging their implementations. They are exact
copies.

Likewise, there's no need to have to I18n keys with almost the same
content.

app/controllers/spree/admin/base_controller.rb

@@ -21,18 +21,6 @@ module Spree
         flash[:notice] = warning if warning.present?
       end
 
-      # This is in Spree::Core::ControllerHelpers::Auth
-      # But you can't easily reopen modules in Ruby
-      def unauthorized
-        if spree_current_user
-          flash[:error] = t(:authorization_failure)
-          redirect_to '/unauthorized'
-        else
-          store_location
-          redirect_to main_app.root_path(anchor: "login?after_login=#{request.env['PATH_INFO']}")
-        end
-      end
-
       protected
 
       def model_class