Virtualbox vagrant-contrib boxes to be retired

[Emmanuel Kasper <manu@debian.org>]

Hi Lucas, Hi Team
I need to retire the vagrant-contrib boxes from the list of Vagrant
boxes that we build. This means no shared folder anymore for Debian 10
Virtualbox boxes (NB: I feel a word redundancy here)

Reason for that is that we used a special dkms to build the guest
extensions from unstable
(https://packages.debian.org/stretch-backports/virtualbox-guest-dkms)
and this is not available anymore in unstable, since Debian 11 and
higher the kernel modules are available in the main kernel tree and the
dkms stuff is not needed anymore.

Lucas, if that fine is for you I will remove the boxes from the build
config next week.

Emmanuel