board: st: remove the nand MTD configuration for NOR boot in stm32mp1 board

board: st: remove the nand MTD configuration for NOR boot in stm32mp1 board

Since commit d5d726d3cc47 ("configs: stm32mp1: only support SD card after
NOR in bootcmd_stm32mp"), the stm32mp1 boards only support SD card after
NOR boot device, so the MTD partitions for nand0 or spi-nand0 are useless
(no need of "UBI" partition in nand0 or spi-nand0).

This patch removes these nand MTD update for nor boot and simplify nand0
and spi-nand0 support (remove the mtd_boot variable).

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>