Remove IncomingMessage#destroy_raw_email

Not called anywhere now. Was used as a workaround for a bug in earlier
versions of Rails. The workaround was removed in 941349d but cleaning
up this method was missed.

app/models/incoming_message.rb

@@ -148,10 +148,6 @@ def update_request
     info_request.update_last_public_response_at
   end
 
-  def destroy_raw_email
-    raw_email.destroy
-  end
-
   # And look up by URL part number and display filename to get an attachment
   # TODO: relies on extract_attachments calling MailHandler.ensure_parts_counted
   # The filename here is passed from the URL parameter, so it's the