Do not list mirrors.txt and the TU Chemnitz ubuntu-ports mirror

The mirror http://ftp.tu-chemnitz.de/pub/linux/ubuntu-ports/ was often the fastest one here, but it's for different architectures only.

That the mirrors.txt file in Ubuntu came up here, was because it had remembered the country code last found in the Mint mirror list when it started to check the Ubuntu mirrors and then thought this "mirror" was in South Africa too (ZA is the latest locale in the Mint mirror list). country_code = None avoids this.