debianutils: Remove postinstall for some apps.

The mkboot and sensible-* apps have been removed from
upstream debianutils.  Remove them from the postinstall
stages.
    https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=197463
    http://en.it-usenet.org/thread/18153/169437/



Signed-off-by: Drew Moseley <drew_moseley@mentor.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>