Remove tools/rgb; the code isn't portable enough (relies upon <ndbm.h>)
Tested on NetBSD 3.1 and Fedora Core 8
(XXX: although the generated .db file has the correct contents on FC8,
it's also 161MiB instead of 64KiB. I think this is a problem in the
build of the db routines in tools/compat, and we'll need to debug
separately.).
Fixes cross-build fallout from PR 36836