user name changes: remove unused reason, status fields.

Remove all infrastructure around approving or rejecting user name
changes. Name changes haven't been moderated for several years.

* Remove status, approver_id, change_reason, and rejection_reason fields.
* Remove approve and reject controller actions.

config/routes.rb

@@ -300,12 +300,7 @@ Rails.application.routes.draw do
       get :search
     end
   end
-  resources :user_name_change_requests do
-    member do
-      post :approve
-      post :reject
-    end
-  end
+  resources :user_name_change_requests, only: [:new, :create, :show, :index]
   resource :user_revert, :only => [:new, :create]
   resources :wiki_pages do
     member do