Clear inverseRecord for deleted belongsTo properly

After a deep dive of the relationship internals together with @sly7-7 we've managed to find that if something observes a belongsTo relationship directly, a delete causes `notifyRecordRelationshipRemoved()` and in turn `notifyPropertyChange()` to get called before `inverseRecord` has been set to `null`, which results in observers getting the old value of the relationship from `getRecord()`.

Fixes #2447