boards: arm: nrf9160dk_nrf9160: disable external flash by default

Disable external flash by default. When enabled, it will require the
SPI driver to be enabled, which in turn require multithreading support.
This adds a lot of overhead to applications not needing the external
flash.

Signed-off-by: Eivind Jølsgard <eivind.jolsgard@nordicsemi.no>