Do not allow adding new API users outside of registrar details view

This feature is not needed.

config/routes.rb

@@ -270,7 +270,7 @@ Rails.application.routes.draw do
     end
 
     resources :admin_users
-    resources :api_users do
+    resources :api_users, except: %i[new] do
       resources :certificates do
         member do
           post 'sign'