board/STm32_Mini_dev_x_defconfig: ARM_MPU enabled without MPU.

This commit resolves warning ARM_MPU & HW_STACK_PROTECTION
during build. The MCU in STm32_Mini_dev (black & blue) has
no MPU support, but MPU was still enabled in defconfigs which
was the reason behind warnings. This commit removes
ARM_MPU & HW_STACK_PROTECTION from defconfig for both
STm32_Mini_dev development boards.

fix: zephyrproject-rtos/zephyr#36408

Signed-off-by: Affrin Pinhero <affrin.pinhero@hcl.com>