stop adding buildd APT entries by default

See e.g. job #14988960:

Trigger:

src:cacti from buster-backports
src:libphp-phpmailer from buster-back...
src:bla from buster-fasttrack

buster-backports added as a extra apt source (this was before extra apt sources were changed to take a fixed list):

autopkgtest [21:13:10]: starting date: 2021-09-02
autopkgtest [21:13:10]: version 5.16
autopkgtest [21:13:10]: host ci-worker-arm64-11; command line: /usr/bin/autopkgtest --no-built-binaries '--setup-commands=echo '"'"'cacti oldstable/arm64'"'"' > /var/tmp/debci.pkg 2>&1 || true' '--setup-commands=echo '"'"'Acquire::Retries "10";'"'"' > /etc/apt/apt.conf.d/75retry 2>&1 || true' --user debci --apt-upgrade '--add-apt-source=deb http://incoming.debian.org/debian-buildd buildd-buster-backports main contrib non-free' --add-apt-release=buster-backports --pin-packages=buster-backports=src:cacti,src:libphp-phpmailer '--add-apt-source=deb http://incoming.debian.org/debian-buildd buildd-buster-fasttrack-proposed-updates main contrib non-free' --add-apt-release=buster-fasttrack --pin-packages=buster-fasttrack=src:bla --output-dir /tmp/tmp.ea5TcFK5gj/autopkgtest-incoming/oldstable/arm64/c/cacti/14988960 cacti -- lxc --sudo --name ci-245-5ef9eca2 autopkgtest-oldstable-arm64
autopkgtest [21:13:29]: @@@@@@@@@@@@@@@@@@@@ test bed setup
Get:1 http://incoming.debian.org/debian-buildd buildd-buster-backports InRelease [24.1 kB]
Fetched 24.1 kB in 1s (39.4 kB/s)
Reading package lists...
Hit:1 http://incoming.debian.org/debian-buildd buildd-buster-backports InRelease
Ign:2 http://incoming.debian.org/debian-buildd buildd-buster-fasttrack-proposed-updates InRelease
Err:3 http://incoming.debian.org/debian-buildd buildd-buster-fasttrack-proposed-updates Release
  404  Not Found [IP: 130.89.148.77 80]
Reading package lists...
E: The repository 'http://incoming.debian.org/debian-buildd buildd-buster-fasttrack-proposed-updates Release' does not have a Release file.
Hit:1 http://incoming.debian.org/debian-buildd buildd-buster-backports InRelease
Ign:2 http://incoming.debian.org/debian-buildd buildd-buster-fasttrack-proposed-updates InRelease
Err:3 http://incoming.debian.org/debian-buildd buildd-buster-fasttrack-proposed-updates Release
  404  Not Found [IP: 149.20.4.15 80]
Reading package lists...
E: The repository 'http://incoming.debian.org/debian-buildd buildd-buster-fasttrack-proposed-updates Release' does not have a Release file.
Hit:1 http://incoming.debian.org/debian-buildd buildd-buster-backports InRelease
Ign:2 http://incoming.debian.org/debian-buildd buildd-buster-fasttrack-proposed-updates InRelease
Err:3 http://incoming.debian.org/debian-buildd buildd-buster-fasttrack-proposed-updates Release
  404  Not Found [IP: 128.31.0.62 80]
Reading package lists...
E: The repository 'http://incoming.debian.org/debian-buildd buildd-buster-fasttrack-proposed-updates Release' does not have a Release file.
autopkgtest [21:14:06]: ERROR: "sh -ec 
        echo "$1" >> /etc/apt/sources.list.d/$2
        for retry in 1 2 3; do
            apt-get --no-list-cleanup \
                    -o Dir::Etc::sourcelist=/etc/apt/sources.list.d/$2 \
                    -o Dir::Etc::sourceparts=/dev/null \
                    update 2>&1 && break || \
                    if [ $retry = 3 ] ; then
                        return 1
                    else
                        sleep 15
                    fi
        done
         sh deb http://incoming.debian.org/debian-buildd buildd-buster-fasttrack-proposed-updates main contrib non-free autopkgtest-add-apt-sources.list" failed with status 1

we are trying to add a buildd-* suite when there is none; also, when using pinning for packages from, say, unstable, you won't get packages from buildd-unstable because that's a different suite. So, the best option here is to stop adding buildd entries at all. They can still be configured in extra APT sources for the use cases where that's needed.

TODO: let the salsa CI team know of this change; maybe they know about people who rely on having the buildd suites enabled