DEV: Drop experimental enable_diffhtml_preview setting (#31306)

This was intended to provide a better UX for interactive elements in the
composer preview. However, the morphing strategy has irreconcilable
conflicts with our `decorateCooked` API, and so we have been unable to
enable this by default.

Going forward, we're focussing efforts on the WYSIWYG composer to
provide this kind of smooth UX, so we're dropping the
`enable_diffhtml_preview` approach.

spec/system/composer/morphed_preview_spec.rb

@@ -1,41 +0,0 @@
-# frozen_string_literal: true
-
-describe "Morphed Composer Preview", type: :system do
-  fab!(:user) { Fabricate(:user, username: "bob", refresh_auto_groups: true) }
-  let(:composer) { PageObjects::Components::Composer.new }
-
-  before do
-    SiteSetting.enable_diffhtml_preview = true
-    sign_in user
-    visit("/new-topic")
-  end
-
-  it "correctly morphs code blocks" do
-    composer.fill_content <<~MD
-      ```js
-      const = {
-        id: t.name,
-        text: t.name,
-        name: t.name,
-      ```
-    MD
-
-    within(composer.preview) { expect(find("code.lang-js")).to have_text("const = {") }
-
-    composer.move_cursor_after("const")
-    composer.type_content("ant")
-
-    within(composer.preview) { expect(find("code.lang-js")).to have_text("constant = {") }
-  end
-
-  it "correctly morphs mentions" do
-    composer.fill_content("@bob text")
-
-    within(composer.preview) { expect(find("a.mention")).to have_text("@bob") }
-
-    composer.select_all
-    composer.type_content("@system")
-
-    within(composer.preview) { expect(find("a.mention")).to have_text("@system") }
-  end
-end

spec/system/composer_uploads_spec.rb

@@ -137,26 +137,6 @@ describe "Uploading files in the composer", type: :system do
       expect(composer).to have_no_in_progress_uploads
       expect(composer.preview).to have_css(".onebox-placeholder-container")
     end
-
-    it "shows video player in composer" do
-      SiteSetting.enable_diffhtml_preview = true
-
-      visit "/new-topic"
-      expect(composer).to be_opened
-      topic.fill_in_composer_title("Video upload test")
-
-      file_path_1 = file_from_fixtures("small.mp4", "media").path
-      attach_file(file_path_1) { composer.click_toolbar_button("upload") }
-
-      expect(composer).to have_no_in_progress_uploads
-      expect(composer.preview).to have_css(".video-container video")
-
-      expect(page).to have_css(
-        ".video-container video source[src]",
-        visible: false,
-        wait: Capybara.default_max_wait_time,
-      )
-    end
   end
 
   context "when multiple images are uploaded" do