cmd: ubi: delete useless and misleading definitions

These definitions are simply not used and are misleading because similar
definitions exist in jffs2/load_kernel.h and are used widely to define
MTD device types (which is, by the way, totally redundant with what the
MTD core does). Remove these definitions.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Jagan Teki <jagan@openedev.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>