Don't create link_set for edition links in factory

We encountered an issue when creating an edition link separately to its
edition (i.e. not using the `links_hash` transient/argument), whereby
both a link set association was created even when explicitly creating an
edition association

Co-authored-by: Mike Patrick <mike.patrick2@digital.cabinet-office.gov.uk>

Author: yndajas

spec/factories/link.rb

@@ -1,6 +1,6 @@
 FactoryBot.define do
   factory :link do
-    link_set
+    link_set          { create(:link_set) unless edition }
     target_content_id { SecureRandom.uuid }
     link_type         { "organisations" }
   end