Revert "fix the case when relation is set to null"

This reverts commit 25548176bb.

src/PersistentData/PersistentDataManager.php

@@ -305,8 +305,6 @@ class PersistentDataManager implements IPersistentDataManager
 
             if ($relationModel !== null) {
                 $model->$methodSet($relationModel->getId());
-            } else {
-                $model->$methodSet(null);
             }
         }
     }