Remove duplicate method

#per_page was defined in two separate pull requests, so here let's leave
the one that returns default per page number.

Author: chaimann

admin/app/controllers/solidus_admin/resources_controller.rb

@@ -96,8 +96,6 @@ def resources_collection
       resource_class.all
     end
 
-    def per_page; end
-
     def set_resource
       @resource ||= resource_class.find(params[:id]).tap do |resource|
           instance_variable_set("@#{resource_name}", resource)