boards: sam: kconfig: Do not assign promptless SOC_FAMILY_SAM 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_SERIES_SAM3X,
SOC_SERIES_SAM4S, and SOC_SERIES_SAME70.

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