boards: exp432p401r: Do not assign promptless HAS_MSP432P4XXSDK symbol

Assignments have no effect on promptless symbols. Flagged by
https://github.com/zephyrproject-rtos/zephyr/pull/20742

.

This symbol is enabled through being selected by SOC_MSP432P401R, in
soc/arm/ti_simplelink/msp432p4xx/Kconfig.soc.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>