samples: minimal: kconfig: Do not assign promptless MEMORY_PROTECTION

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

.

This symbol is select'ed automatically by the X86_MMU, ARM_MPU, and
ARC_MPU symbols.

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