Remove unused load_promotion_category method and callback

Removed the load_promotion_category method and its corresponding 
before_action for the non-existent :move action.

admin/app/controllers/solidus_admin/promotion_categories_controller.rb

@@ -4,8 +4,6 @@ module SolidusAdmin
   class PromotionCategoriesController < SolidusAdmin::BaseController
     include SolidusAdmin::ControllerHelpers::Search
 
-    before_action :load_promotion_category, only: [:move]
-
     def index
       promotion_categories = apply_search_to(
         Spree::PromotionCategory.all,
@@ -27,12 +25,5 @@ def destroy
       flash[:notice] = t('.success')
       redirect_back_or_to promotion_categories_path, status: :see_other
     end
-
-    private
-
-    def load_promotion_category
-      @promotion_category = Spree::PromotionCategory.find(params[:id])
-      authorize! action_name, @promotion_category
-    end
   end
 end