kconfig: Remove assignments to promptless SOC_FAMILY_ARM symbol

SOC_FAMILY_ARM has no prompt. Assignments in configuration files have no
effect on symbols without prompts. A prompt means the symbol is
user-configurable.

SOC_FAMILY_ARM is instead enabled indirectly through being selected by
other symbols.

Detected through some work-in-progress improved error checking.

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