Use sitemap service instead of static files

This PR

- Removes the static sitemap files
- Removes the scripts and artefacts to create the static sitemap files
- Uses the sitemap service from the backend to serve the sitemap files (https://github.com/geoadmin/mf-chsdi3/pull/874)

Note: this uses https://github.com/geoadmin/mf-chsdi3/pull/874 and should be merged after that. For testing make sure your api_url setting points to an instance with the above PR applied.

This is in preparation to replace https://github.com/geoadmin/mf-geoadmin3/pull/1448