[comboot] Remove COMBOOT image support by default

iPXE's support for COMBOOT images is now quite outdated; it has not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>

src/config/defaults/pcbios.h

 #define	IMAGE_PXE		/* PXE image support */
 #define IMAGE_SCRIPT		/* iPXE script image support */
 #define IMAGE_BZIMAGE		/* Linux bzImage image support */
-#define IMAGE_COMBOOT		/* SYSLINUX COMBOOT image support */
 
 #define PXE_STACK		/* PXE stack in iPXE - required for PXELINUX */
 #define PXE_MENU		/* PXE menu booting */