cmd: ubi: clean the partition handling

UBI should not mess with MTD partitions, now that the partitions are
handled in a clean way, clean the ubi command and avoid using this
uneeded extra-glue to reference the devices.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>