Remove suggested_ordered_related_items

Now that the [related links recommender](https://github.com/alphagov/govuk-related-links-recommender)
is no longer running, suggested_ordered_related_items will be growing
increasingly stale.

I'm not sure whether publishing-api is even clever enough to keep
suggested_ordered_related_items links somewhat up to date (e.g. if one
is withdrawn, unpublished, or if a slug is changed) - it doesn't look
like it from [a cursory glance at the
code](https://github.com/alphagov/publishing-api/blob/main/lib/expansion_rules.rb).

Note that there are around 164,000 published editions with
suggested_ordered_related_items and only around 2,500 published editions
with ordered_related_items, so this will have the effect of removing the
related links sidebar from the majority of pages on GOV.UK (around
161,000 of them).

app/controllers/content_items_controller.rb

@@ -152,9 +152,7 @@ def load_content_item
   def ordered_related_items(links)
     return [] if links["ordered_related_items_overrides"].present?
 
-    links["ordered_related_items"].presence || links.fetch(
-      "suggested_ordered_related_items", []
-    )
+    links["ordered_related_items"]
   end
 
   def format_banner_links(links, type)

test/controllers/content_items_controller_test.rb

@@ -153,16 +153,6 @@ class ContentItemsControllerTest < ActionController::TestCase
     assert_nil content_item["links"]["ordered_related_items"]
   end
 
-  test "gets item from content store and replaces ordered_related_items there are no existing links or overrides" do
-    content_item = content_store_has_schema_example("case_study", "case_study")
-
-    get :show, params: { path: path_for(content_item) }
-    assert_response :success
-    assert_empty content_item["links"]["ordered_related_items"], "Content item should not have existing related links"
-    assert_not_empty content_item["links"]["suggested_ordered_related_items"], "Content item should have existing suggested related links"
-    assert_equal assigns[:content_item].content_item["links"]["ordered_related_items"], content_item["links"]["suggested_ordered_related_items"]
-  end
-
   test "sets the expiry as sent by content-store" do
     content_item = content_store_has_schema_example("case_study", "case_study")
     stub_content_store_has_item(content_item["base_path"], content_item, max_age: 20)