configs/stm32mp157: remove extra metadata_csum mkfs option

configs/stm32mp157: remove extra metadata_csum mkfs option

Since commit [1] in U-Boot upstrea, there is no necessity to pass extra
metadata_csum option due to changed env location in U-Boot so we can
drop it completely.

[1] 76db1681da52342ca9f4fb7e6787bd83cc82f429:
stm32mp1: use a specific SD/eMMC partition for U-Boot enviromnent

Signed-off-by: Bartosz Bilas <b.bilas@grinn-global.com>
[yann.morin.1998@free.fr: 76db1681d referecnes a U-Boot commit]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>