drivers/flash/flash_ite_i8xxx2: remove write_protection handler

The handler was reworked to internal function and it
is called from the erase and the write
procedures automatically now.

This change was made due to deprecation of the flash write-protection
API.

Explanation for so late removal:
Reworked callback was introduced despite that the API had been
already deprecated at the addition time.

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