FIX: Do not error _removeDeleteOnOwnerReplyBookmarks on navigate (PR #14426)

When creating a reply after already navigating out of the topic (e.g. open the reply composer, go to a different topic, then create the post), the _removeDeleteOnOwnerReplyBookmarks function was erroring because it relied on the topic model being present.

We can skip this function altogether if the topic model is not present, because the PostCreator already takes care of deleting bookmarks with the on_owner_reply auto_delete_preference. The _removeDeleteOnOwnerReplyBookmarks function just cleans up the in-memory post stream and topic model.