Ensure messageCache is updated when properties are unset

Co-authored-by: trevor-signal <131492920+trevor-signal@users.noreply.github.com>

ts/util/cleanup.ts

@@ -89,7 +89,7 @@ async function cleanupStoryReplies(
           reply,
           'cleanupStoryReplies/1:1'
         );
-        model.unset('storyReplyContext');
+        model.set('storyReplyContext', undefined);
         await model.hydrateStoryContext(story, { shouldSave: true });
       })
     );