ConversationTree is now a plugin (not oldschool setting)

Conversation trees works pretty bad with the current layout, javascript
etc. So it's best if we separate it and work on it as a side-project. The
oldschool settings are currently being deprecated (or broken out like this).

I'll wait with removing User preferences for oldschool conversation tree,
since that might be reusable data. But I guess it will go in the near future.

classes/User.php

@@ -981,18 +981,6 @@ class User extends Managed_DataObject
         return false;
     }
 
-    function conversationTree()
-    {
-        if (common_config('oldschool', 'enabled')) {
-            $osp = Old_school_prefs::getKV('user_id', $this->id);
-            if (!empty($osp)) {
-                return $osp->conversation_tree;
-            }
-        }
-
-        return false;
-    }
-
     function streamNicknames()
     {
         if (common_config('oldschool', 'enabled')) {