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>

Dockerfile.frontend

@@ -5,10 +5,6 @@ ARG WORKDIR=/tmp/sources
 # Skip tests
 ARG DEB_BUILD_OPTIONS=nocheck
 
-# Use stretch since it’s not the stable anymore
-RUN \
- sed -i 's/stable/stretch/g' /etc/apt/sources.list
-
 RUN apt-get update \
  && apt-get install -y apt-transport-https ca-certificates build-essential