Remove LDSignature on actor Delete activities (#21466)

They are currently not used for anything and represent more than half of the
payload size.

app/services/delete_account_service.rb

@@ -267,7 +267,7 @@ class DeleteAccountService < BaseService
   end
 
   def delete_actor_json
-    @delete_actor_json ||= Oj.dump(serialize_payload(@account, ActivityPub::DeleteActorSerializer, signer: @account, always_sign: true))
+    @delete_actor_json ||= Oj.dump(serialize_payload(@account, ActivityPub::DeleteActorSerializer))
   end
 
   def delivery_inboxes