Do not query database for unread notifications if all are retrieved

Even though the unread count is known and we select all info on notifications in a previous query, we query the database again for notifications to ensure that latest read notifications show (defaults to a maximum of 5): https://github.com/phpbb/phpbb/blob/3.1.x/phpBB/phpbb/notification/manager.php#L207

This is however not needed if we already selected all unread notifications in the previous query.