Remove code relating to indexing topical event about pages in Search API

These pages were not being indexed by Whitehall because the `can_index_in_search?` method was evaluating to false. This was due to topical event about pages being absent from the search API presenter's allowlist.

Given nobody seems to have missed them, it seems reasonable to maintain the status quo. We may choose to start indexing them via Publishing API in future, but we don't want to reintroduce the behaviour to Whitehall.

Author: ryanb-gds

app/models/topical_event_about_page.rb

@@ -1,5 +1,4 @@
 class TopicalEventAboutPage < ApplicationRecord
-  include Searchable
   include PublishesToPublishingApi
 
   belongs_to :topical_event
@@ -14,19 +13,6 @@ class TopicalEventAboutPage < ApplicationRecord
 
   after_commit :republish_topical_event_to_publishing_api
 
-  searchable title: :name,
-             link: :search_link,
-             content: :indexable_content,
-             description: :summary
-
-  def search_link
-    base_path
-  end
-
-  def indexable_content
-    Govspeak::Document.new(body).to_text
-  end
-
   def base_path
     "/government/topical-events/#{topical_event.slug}/about"
   end

test/unit/app/models/topical_event_about_page_test.rb

@@ -1,13 +1,6 @@
 require "test_helper"
 
 class TopicalEventAboutPageTest < ActiveSupport::TestCase
-  test "should return search index data suitable for Searchable" do
-    event = create(:topical_event)
-    page = create(:topical_event_about_page, topical_event: event)
-    assert_equal page.name, page.search_index["title"]
-    assert_equal "/government/topical-events/#{event.slug}/about", page.search_index["link"]
-  end
-
   test "public_path returns the correct path" do
     object = create(:topical_event, slug: "foo", topical_event_about_page: create(:topical_event_about_page))
     assert_equal "/government/topical-events/foo/about", object.topical_event_about_page.public_path