Revert "Preparation for install python-stem, which is unfortunately only available from Debian testing, not stable. Added Debian testing repository to /etc/apt/sources.list and configured apt pinning in /etc/apt/preferences.d/50_whonix. #60"

This reverts commit b7acdf91d31db38eb56bdeb3756241e3b3b5d25f.
added pandoc (required for whonixcheck as html2text converter)