Remove the topic_featured_link_onebox setting. We will always try to onebox a link and add it to the body if topic_featured_link_enabled is enabled.

app/assets/javascripts/discourse/components/composer-body.js.es6

@@ -13,8 +13,7 @@ export default Ember.Component.extend({
                       'composer.canEditTitle:edit-title',
                       'composer.createdPost:created-post',
                       'composer.creatingTopic:topic',
-                      'composer.whisper:composing-whisper',
-                      'composer.showComposerEditor::topic-featured-link-only'],
+                      'composer.whisper:composing-whisper'],
 
   @computed('composer.composeState')
   composeState(composeState) {

app/assets/javascripts/discourse/models/composer.js.es6

@@ -195,11 +195,6 @@ const Composer = RestModel.extend({
 
   }.property('action', 'post', 'topic', 'topic.title'),
 
-  @computed('canEditTopicFeaturedLink')
-  showComposerEditor(canEditTopicFeaturedLink) {
-    return canEditTopicFeaturedLink ? !this.siteSettings.topic_featured_link_onebox : true;
-  },
-
   // whether to disable the post button
   cantSubmitPost: function() {
 

config/site_settings.yml

@@ -436,9 +436,6 @@ posting:
   topic_featured_link_enabled:
     client: true
     default: false
-  topic_featured_link_onebox:
-    client: true
-    default: false
   open_topic_featured_link_in_external_window:
     client: true
     default: true

lib/post_creator.rb

@@ -104,11 +104,6 @@ class PostCreator
       end
     end
 
-    onebox_featured_link = SiteSetting.topic_featured_link_enabled && SiteSetting.topic_featured_link_onebox && guardian.can_edit_featured_link?(find_category_id)
-    if onebox_featured_link
-      @opts[:raw] = DiscourseFeaturedLink.cache_onebox_link(@opts[:featured_link])
-    end
-
     setup_post
 
     return true if skip_validations?
@@ -122,7 +117,7 @@ class PostCreator
     DiscourseEvent.trigger :before_create_post, @post
     DiscourseEvent.trigger :validate_post, @post
 
-    post_validator = Validators::PostValidator.new(skip_topic: true, skip_post_body: onebox_featured_link)
+    post_validator = Validators::PostValidator.new(skip_topic: true)
     post_validator.validate(@post)
 
     valid = @post.errors.blank?

lib/post_revisor.rb

@@ -102,13 +102,6 @@ class PostRevisor
 
       topic_changes.record_change('featured_link', topic_changes.topic.featured_link, featured_link)
       topic_changes.topic.featured_link = featured_link
-
-      if SiteSetting.topic_featured_link_onebox
-        post = topic_changes.topic.first_post
-        post.raw = DiscourseFeaturedLink.cache_onebox_link(featured_link)
-        post.save!
-        post.rebake!
-      end
     end
   end
 

spec/components/post_creator_spec.rb

@@ -20,7 +20,7 @@ describe PostCreator do
     let(:creator_with_category) { PostCreator.new(user, basic_topic_params.merge(category: category.id )) }
     let(:creator_with_meta_data) { PostCreator.new(user, basic_topic_params.merge(meta_data: {hello: "world"} )) }
     let(:creator_with_image_sizes) { PostCreator.new(user, basic_topic_params.merge(image_sizes: image_sizes)) }
-    let(:creator_with_featured_link) { PostCreator.new(user, title: "featured link topic", archetype_id: 1, featured_link: "http://discourse.org") }
+    let(:creator_with_featured_link) { PostCreator.new(user, title: "featured link topic", archetype_id: 1, featured_link: "http://www.discourse.org", raw: "http://www.discourse.org") }
 
     it "can create a topic with null byte central" do
       post = PostCreator.create(user, title: "hello\u0000world this is title", raw: "this is my\u0000 first topic")
@@ -244,12 +244,10 @@ describe PostCreator do
         end
       end
 
-      it 'creates a post without raw' do
+      it 'creates a post with featured link' do
         SiteSetting.topic_featured_link_enabled = true
-        SiteSetting.topic_featured_link_onebox = true
         post = creator_with_featured_link.create
-        expect(post.topic.featured_link).to eq('http://discourse.org')
-        expect(post.raw).to eq('http://discourse.org')
+        expect(post.topic.featured_link).to eq('http://www.discourse.org')
       end
 
       describe "topic's auto close" do

test/javascripts/models/composer-test.js.es6

@@ -41,9 +41,9 @@ test('missingReplyCharacters', function() {
   missingReplyCharacters('hi', false, true,  Discourse.SiteSettings.min_first_post_length - 2, 'too short first post');
   missingReplyCharacters('hi', true, false,  Discourse.SiteSettings.min_private_message_post_length - 2, 'too short private message');
 
-  Discourse.SiteSettings.topic_featured_link_onebox = true;
-  const composer = createComposer({ canEditTopicFeaturedLink: true });
-  equal(composer.get('missingReplyCharacters'), 0, "don't require any post content");
+  // TODO: test that presence of featured link makes this test pass
+  // const composer = createComposer({ canEditTopicFeaturedLink: true });
+  // equal(composer.get('missingReplyCharacters'), 0, "don't require any post content");
 });
 
 test('missingTitleCharacters', function() {