Bug 5468: removin apt-get-debian-lenny and updating INSTALL.debian

Bug 5468: removin apt-get-debian-lenny and updating INSTALL.debian

install_misc/apt-get-debian-lenny.sh: obsolete, causing problems and a questionable practice, so deleted INSTALL.debian: updated to use debian.packages instead and cite more related docs

Amended following feedback comment #6 from Robin Sheat <robin@catalyst.net.nz>

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
(cherry picked from commit a9d24af4700b9b1e384be6406987b43b80095e1c)

Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>