[Dockerfile] Remove apt src disabling, no longer needed

[PeterDaveHello]

Looks like it's no longer needed. All commented by default. Can verify by the command below:

$ for ver in 16.04 18.04 20.04 22.04; do docker run --rm -it ubuntu:$ver sh -c 'grep deb-src /etc/apt/sources.list'; echo; done
# deb-src http://archive.ubuntu.com/ubuntu/ xenial main restricted
# deb-src http://archive.ubuntu.com/ubuntu/ xenial-updates main restricted
# deb-src http://archive.ubuntu.com/ubuntu/ xenial universe
# deb-src http://archive.ubuntu.com/ubuntu/ xenial-updates universe
# deb-src http://archive.ubuntu.com/ubuntu/ xenial multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ xenial-updates multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse
# deb-src http://archive.canonical.com/ubuntu xenial partner
# deb-src http://security.ubuntu.com/ubuntu/ xenial-security main restricted
# deb-src http://security.ubuntu.com/ubuntu/ xenial-security universe
# deb-src http://security.ubuntu.com/ubuntu/ xenial-security multiverse

# deb-src http://archive.ubuntu.com/ubuntu/ bionic main restricted
# deb-src http://archive.ubuntu.com/ubuntu/ bionic-updates main restricted
# deb-src http://archive.ubuntu.com/ubuntu/ bionic universe
# deb-src http://archive.ubuntu.com/ubuntu/ bionic-updates universe
# deb-src http://archive.ubuntu.com/ubuntu/ bionic multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ bionic-updates multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ bionic-backports main restricted universe multiverse
# deb-src http://archive.canonical.com/ubuntu bionic partner
# deb-src http://security.ubuntu.com/ubuntu/ bionic-security main restricted
# deb-src http://security.ubuntu.com/ubuntu/ bionic-security universe
# deb-src http://security.ubuntu.com/ubuntu/ bionic-security multiverse

# deb-src http://archive.ubuntu.com/ubuntu/ focal main restricted
# deb-src http://archive.ubuntu.com/ubuntu/ focal-updates main restricted
# deb-src http://archive.ubuntu.com/ubuntu/ focal universe
# deb-src http://archive.ubuntu.com/ubuntu/ focal-updates universe
# deb-src http://archive.ubuntu.com/ubuntu/ focal multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ focal-updates multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse
# deb-src http://archive.canonical.com/ubuntu focal partner
# deb-src http://security.ubuntu.com/ubuntu/ focal-security main restricted
# deb-src http://security.ubuntu.com/ubuntu/ focal-security universe
# deb-src http://security.ubuntu.com/ubuntu/ focal-security multiverse

# deb-src http://archive.ubuntu.com/ubuntu/ jammy main restricted
# deb-src http://archive.ubuntu.com/ubuntu/ jammy-updates main restricted
# deb-src http://archive.ubuntu.com/ubuntu/ jammy universe
# deb-src http://archive.ubuntu.com/ubuntu/ jammy-updates universe
# deb-src http://archive.ubuntu.com/ubuntu/ jammy multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ jammy-updates multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ jammy-backports main restricted universe multiverse
# deb-src http://security.ubuntu.com/ubuntu/ jammy-security main restricted
# deb-src http://security.ubuntu.com/ubuntu/ jammy-security universe
# deb-src http://security.ubuntu.com/ubuntu/ jammy-security multiverse

[ljharb]

I'll take your word for it, thanks :-)