You have to log into your server and edit the file, which is located by default in /var/discourse/containers/app.yml
Any text editor will do.
Near the end of the file contents, you should see this line:
- git clone https://github.com/discourse/discourse-sitemap
Delete this line, and rebuild from the command line (./launcher rebuild app), or update your forum through the interface.
edit: oh, 1 minute late!