samples: Bluetooth: Mesh: remove nrf53_ns settings related workaround

The issue with non secure storage system has been fixed
in nrf53_ns platform: https://github.com/zephyrproject-rtos/zephyr/issues/59376


No need workarounds anymore.

Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>