Remove unused 'link_reportable_type' / 'link_reportable_id' fields

Following on from #9986 which introduces the 'edition_id' field,
we no longer need the old polymorphic fields.

db/migrate/20250226115201_remove_link_reportable_fields_from_link_checker_api_reports.rb

@@ -0,0 +1,28 @@
+class RemoveLinkReportableFieldsFromLinkCheckerApiReports < ActiveRecord::Migration[8.0]
+  def up
+    # Catch any link reports that have been created since
+    # `AddEditionIdToLinkCheckerApiReports` was run.
+    execute <<-SQL.squish
+      UPDATE link_checker_api_reports
+      SET edition_id = link_reportable_id
+      WHERE link_reportable_type = 'Edition' AND edition_id IS NULL
+    SQL
+
+    remove_index :link_checker_api_reports, name: "index_link_checker_api_reportable"
+    remove_column :link_checker_api_reports, :link_reportable_type
+    remove_column :link_checker_api_reports, :link_reportable_id
+  end
+
+  def down
+    add_column :link_checker_api_reports, :link_reportable_type, :string
+    add_column :link_checker_api_reports, :link_reportable_id, :integer
+    add_index :link_checker_api_reports, [:link_reportable_type, :link_reportable_id], name: "index_link_checker_api_reportable"
+
+    # Restore data
+    execute <<-SQL.squish
+      UPDATE link_checker_api_reports
+      SET link_reportable_type = 'Edition', link_reportable_id = edition_id
+      WHERE edition_id IS NOT NULL
+    SQL
+  end
+end