Fixes https://github.com/vector-im/element-web/issues/15559 This isn't exactly perfect as an implementation: if the user refreshes immediately after forgetting then there is a good chance the room re-appears because of the sync accumulator. At the very least this change makes it so in *most* cases the room goes away, which is probably good enough until https://github.com/vector-im/element-web/issues/14038 can be implemented properly.