Remove unused 'search_index' method in Edition::TopicalEvents

This looks like it would only be used by content types that update
Search API directly. But all classes that include Edition::TopicalEvents
inherit from Edition, which updates Search API the idiomatic way
via Publishing API, so I don't think this method is doing anything.

Author: ChrisBAshton

app/models/concerns/edition/topical_events.rb

@@ -23,8 +23,4 @@ def process_associations_before_save(edition)
   def can_be_associated_with_topical_events?
     true
   end
-
-  def search_index
-    super.merge("topical_events" => topical_events.pluck(:slug))
-  end
 end