Only include redirects in sitemaps

It's not helpful to have entries which point at 404s or 410s. Including redirescts meant that rendering took longer and we were more likely to reach the 50k URL limit for sitemaps.

This more or less copies the behaviour of the Redirector: https://github.com/alphagov/redirector/blob/master/tools/generate_sitemap.pl#L33

This should also fix the only failing smokey test for Bouncer.