d/preinst.ex: delete file

Delete unused file debian/preinst.ex.

Signed-off-by: Arturo Borrero Gonzalez <aborrero@cica.es>
dd/README.Debian: delete file

Delete unused README.Debian file.

In the future, maybe this file will be used to distribute additional info
on using fw-admin.

Signed-off-by: Arturo Borrero Gonzalez <aborrero@cica.es>