apt: Remove old Debian stretch stuff

Signed-off-by: Maximilian Wilhelm <max@sdn.clinic>

apt/ffho.list.Debian.stretch

@@ -1,2 +0,0 @@
-deb	http://apt.ffho.net stretch contrib non-free main
-deb-src	http://apt.ffho.net stretch contrib non-free main

apt/salt.list.Debian.stretch

@@ -1 +0,0 @@
-deb http://repo.saltstack.com/py3/debian/9/amd64/3002/ stretch main

apt/sources.list.Debian.stretch

@@ -1,17 +0,0 @@
-#
-# /etc/apt/sources.list (Salt managed)
-#
-
-deb http://deb.debian.org/debian/ stretch main non-free contrib
-deb-src http://deb.debian.org/debian/ stretch main non-free contrib
-
-deb http://security.debian.org/debian-security stretch/updates main contrib non-free
-deb-src http://security.debian.org/debian-security stretch/updates main contrib non-free
-
-# stretch-updates, previously known as 'volatile'
-deb http://deb.debian.org/debian/ stretch-updates main contrib non-free
-deb-src http://deb.debian.org/debian/ stretch-updates main contrib non-free
-
-# stretch-backports, previously on backports.debian.org
-deb http://deb.debian.org/debian/ stretch-backports main contrib non-free
-deb-src http://deb.debian.org/debian/ stretch-backports main contrib non-free

apt/sources.list.Raspbian.stretch

@@ -1,6 +0,0 @@
-#
-# /etc/apt/sources.list (Salt managed)
-#
-
-deb http://debian.bio.lmu.de/raspbian/raspbian/ stretch main contrib non-free rpi
-deb-src http://debian.bio.lmu.de/raspbian/raspbian/ stretch main contrib non-free rpi