boards: minnowboard: kconfig: Do not assign promptless CPU_ATOM 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_ATOM in
soc/x86/atom/Kconfig.soc.

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