{sys,dev}root: Clean up apt lists and caches

Since 7252fff4 "Enable building images out of snapshots switching to live APT repos at the end" we change the APT sources configuration.

This means that the cached contents /var/lib/apt/lists no longer match the what is in /etc/apt/sources.lists so to use apt it is now always required to run apt update before doing anything.

Drop the deadweight, saving approx. 20MB (uncompressed).