Bug#821862: Do not release with stretch

[Andriy Grytsenko]

Well, while your point is right, removal of system-tools-backends will
broke LXDE usability because there is no other alternative package to
manage users - other known users management tools are integrated into
GNOME and KDE, and gnome-system-tools is only standalone package which
can be used in any other DE. Yes, literally, there is no alternative to
the gnome-system-tools and it depends on system-tools-backends.