Only include redirects in sitemaps

It's not helpful to have entries which point at 404s or 410s. It also 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.