This reverts commit
47c2f47352507002d023a7dd9e1098e3c42257da.
fluxbox is back in Debian/testing (v1.3.5-2.1) thanks to an NMU,
while openbox technically works, its desktop "integration" isn't
as smooth as the one we have for fluxbox (and our users are used to
get fluxbox), so let's revert back to fluxbox for now.
See https://github.com/grml/grml-desktop/issues/5