iPXE's support for COMBOOT images is now quite outdated; it has not
kept up to date with changes in the COMBOOT API.  The primary use for
COMBOOT seems to be for menuing support.  Now that we have native iPXE
script-based menus, COMBOOT support can be gracefully retired (with
immense thanks to Daniel Verkamp for having successfully implemented
such an ambitious feature many years ago).

Signed-off-by: Michael Brown <mcb30@ipxe.org>