Hiya! 
That is installed as a plugin. If you see the guide for installing plugins at Install plugins on a self-hosted site, the way to remove the plugin will be to edit your app.yml and remove the line with the discourse-sitemap plugin. 
Hiya!
That is installed as a plugin. If you see the guide for installing plugins at Install plugins on a self-hosted site, the way to remove the plugin will be to edit your app.yml and remove the line with the discourse-sitemap plugin.