Clean default sources.list file for Ubuntu 20.04 focal

deb http://de.archive.ubuntu.com/ubuntu/ focal main restricted universe multiverse
deb http://de.archive.ubuntu.com/ubuntu/ focal-updates main restricted universe multiverse
deb http://de.archive.ubuntu.com/ubuntu/ focal-security main restricted universe multiverse
deb http://de.archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse

deb-src http://de.archive.ubuntu.com/ubuntu/ focal main restricted universe multiverse
deb-src http://de.archive.ubuntu.com/ubuntu/ focal-updates main restricted universe multiverse
deb-src http://de.archive.ubuntu.com/ubuntu/ focal-security main restricted universe multiverse
deb-src http://de.archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse

deb http://archive.canonical.com/ubuntu focal partner
deb-src http://archive.canonical.com/ubuntu focal partner