test: kernel: remove workaround for arm_mpu (keep for nxp_mpu)

The generation of Stack Corruptions reports is, now, supported
in ARM SOCs with the ARM MPU (CONFIG_ARM_MPU). Therefore, this
commit removes the workaround for ARM architecture in
tests/kernel/fatal/ and keeps it only for SOCs with the NXP MPU
(CONFIG_MXU_MPU).

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