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.

lib/spree/core/controller_helpers/auth.rb

@@ -25,7 +25,7 @@ module Spree
         # For example, a popup window might simply close itself.
         def unauthorized
           if spree_current_user
-            flash[:error] = Spree.t(:authorization_failure)
+            flash[:error] = t(:authorization_failure)
             redirect_to '/unauthorized'
           else
             store_location