Drop sources.list fixup since Docker images for stretch use stretch

The fixup is no longer needed:

    $ docker run -it debian:stretch-slim /bin/bash
    root@35839f6c2f2c:/# egrep '^deb.*stretch|stable' /etc/apt/sources.list
    deb http://deb.debian.org/debian stretch main
    deb http://security.debian.org/debian-security stretch/updates main
    deb http://deb.debian.org/debian stretch-updates main

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>
2 jobs for build-backend-image in 5 minutes and 36 seconds (queued for 4 seconds)