FIX: relies only on message bus to set tracking state (#20785)

This manual set was happening only after the request so was not much faster than just waiting on message bus update. It's not by itself changing any behavior or fixing any bug but it makes reasoning about the whole state easier as it happens in only one central place.

plugins/chat/assets/javascripts/discourse/models/chat-channel.js

@@ -172,8 +172,6 @@ export default class ChatChannel extends RestModel {
     // class not to use RestModel
     return ajax(`/chat/api/channels/${this.id}/read/${messageId}`, {
       method: "PUT",
-    }).then(() => {
-      this.currentUserMembership.last_read_message_id = messageId;
     });
   }
 }