arch: arm: remove redundant dependence on ARM_CORE_MPU

MPU_STACK_GUARD option has a direct dependence on ARM_CORE_MPU.
Therefore, it is not required to have a conditional selection
of the option (if ARM_CORE_MPU) in ARM_STACK_PROTECTION.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>