drivers/flash: remove NRF flash protection emulation

NRF devices hardware has flash protection which doesn't
reflect flash API definition well. So fare protection
mechanism was emulated by the software and the driver deals
with hardware flash protection on its own.
Recent change to protection behavior requirement allows
to remove flash API behavior emulation at all.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>