Comment 0 for bug 1855354

[impact]
This breaks apt update in a trusty docker image that does not have ESM enabled.

[test case]
Build a livefs with image_format=plain, project=ubuntu-base. Unpack it, do the usual pre-chroot setup dance (copy resolv.conf, bind mount proc and dev) chroot into it and run apt update.

[regression potential]
The change could conceivably leave apt preference files we want deleted in an image. But this seems very unlikely.