cmd: mtdparts: remove mandatory 'mtdparts=' prefix

All U-Boot users must define the mtdparts environment variable with:
setenv mtdparts mtdparts=...

While this may ease the partition declaration job to be passed to
Linux, this is a pure software limitation and forcing this prefix is a
complete non-sense. Let the user to declare manually the mtdparts
variable without the prefix.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Jagan Teki <jagan@openedev.com>
Reviewed-by: Stefan Roese <sr@denx.de>