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

config/locales/en.yml

@@ -3093,7 +3093,6 @@ See the %{link} to find out more about %{sitename}'s features and to start using
     adjustment: "Adjustment"
     all: "All"
     associated_adjustment_closed: "Associated adjustment closed"
-    authorization_failure: "Authorization failure"
     back_to_adjustments_list: "Back to adjustments"
     back_to_users_list: "Back to users"
     back_to_zones_list: "Back to zones"

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