Only set delete field when deleted_at set on notification

We only want to pre-populate the 'delete' if we know this is in fact an exist event for a deleted instance. Otherwise, there is a chance we may populate it for a previous period's exist. This can happen when an instance is deleted immediately after an audit period ends, which creates a ton of traffic due to instance.exists events. With all of this traffic, the delete can beat to exists event to stacktach and will get stored in the database.