Remove thumb_url from Rocket.Chat

Results in broken thumbnail appearing in all posts.

lib/discourse_chat/provider/rocketchat/rocketchat_provider.rb

@@ -35,8 +35,7 @@ module DiscourseChat::Provider::RocketchatProvider
       text: post.excerpt(SiteSetting.chat_integration_rocketchat_excerpt_length, text_entities: true, strip_links: true, remap_emoji: true),
       mrkdwn_in: ["text"],
       title: "#{topic.title} #{(category == '[uncategorized]') ? '' : category} #{topic.tags.present? ? topic.tags.map(&:name).join(', ') : ''}",
-      title_link: post.full_url,
-      thumb_url: post.full_url
+      title_link: post.full_url
     }
 
     message[:attachments].push(summary)