toradex: verdin-imx8mm/imx8mp: Remove bootcmd_mfg

The bootcmd_mfg env variable is legacy from IMX downstream u-boot branch
and is not needed on mainline.

Signed-off-by: Joao Paulo Goncalves <joao.goncalves@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>