cmd: ubi: Remove useless call to mtdparts_init()

Commit c58fb2cd ("cmd: ubi: clean the partition handling")
introduced a call to mtd_probe_devices() in the ubi_attach() path
and this function takes care of parsing mtdparts/mtdids and
creating/registering the associated mtd partitions.

The mtdparts_init() call in the ubi_detach() path is not only
unnecessary but can sometimes print error messages even when things
work properly (that's the case with SPI NAND devices that have not
been probed with 'mtd list'), which is misleading.

Remove this call to mtdparts_init() and drop the dependency on
CMD_MTDPARTS.

Fixes: c58fb2cd ("cmd: ubi: clean the partition handling")
Reported-by: Stefan Roese <sr@denx.de>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Tested-by: Stefan Roese <sr@denx.de>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Jagan Teki <jagan@openedev.com>