arch: arm: mpu: remove init of static regions from arm_mpu_init()

This commit removes the initialization of static memory regions
from arm_mpu_init(), since they are now initialized, directly,
by the kernel. The programming of MPU regions configured in SOC
definition is kept here.

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