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.

test/factories/user_name_change_request.rb

@@ -1,6 +1,7 @@
 FactoryBot.define do
   factory(:user_name_change_request) do
+    user
+    original_name {FFaker::Internet.user_name}
     desired_name {FFaker::Internet.user_name}
-    change_reason {FFaker::Lorem.sentence}
   end
 end