boards: stm32: kconfig: Do not assign promptless SOC_FAMILY_STM32

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

.

This symbol is enabled through being select'ed by other symbols.

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