boards: arm: nordic nrf91* nrf53* nrf52* nrf51*: Remove scratch areas

The scratch partition is not needed since MCUboot now operates in
swap using move mode instead of swap with scratch, as a result, the
main partitions on Nordic nRF51, nRF52, nRF53 and nRF91 boards can be
expanded to help in fitting large applications to them.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>