boards: cy8ckit: kconfig: Do not assign promptless SOC_FAMILY_PSOC6

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

.

This symbol is selected by SOC_SERIES_PSOC62, in
soc/arm/cypress/psoc6/Kconfig.series.

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