Remove secret location from check-mirrors

Some generous contributor proposed us an hidden location to host our ISO
image. This ISO image should be accessible even in place where
censorship could try to prevent people from downloading Tails.

Before making that script public this information needs to be taken out
of this script. Ideally the script should still be able to verify those
mirrors, only the specific location should be kept in a separate file.

Related issues

• Related to #7667 (closed)

Original created by @sajolida on 7666 (Redmine)