uboot-mvebu: remove enabled CONFIG_CMD_SETEXPR

uboot-mvebu: remove enabled CONFIG_CMD_SETEXPR

We don't need to make sure that we want to have enabled
CONFIG_CMD_SETEXPR by default, since this is already done in U-boot [1].
This was actually needed only for clearfog board [2], which was added in
commit: da0005a6d08ae33d958a6d8a6c0c12dc07b5b2b8 ("uboot-mvebu: add
patch to enable setexpr for clearfog boards) and send to U-boot to fix
it properly. After a while, there was added support for Turris Omnia,
which uses setexpr as well [3], but for this board, there are no fixes
needed in U-boot and that's why we can remove this option here.

It is helpful with shell scripting. If some downstream distributions are
using it, they should correct it in defconfig for related boards.

[1] https://source.denx.de/u-boot/u-boot/-/blob/e95afa56753cebcd20a5114b6d121f281b789006/cmd/Kconfig#L1504

[2] https://github.com/openwrt/openwrt/blob/852126680e21edc71c0c66561ae5a6d7479dcc67/target/linux/mvebu/image/clearfog.bootscript#L7

[3] https://github.com/openwrt/openwrt/blob/852126680e21edc71c0c66561ae5a6d7479dcc67/target/linux/mvebu/image/turris-omnia.bootscript#L2

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>