ITE: drivers/flash: remove flash write protection API

This API has be removed in Zephyr 2.8, and does not need to be used
as a static function. Because flash_it8xxx2_write_protection(false)
is not supported. If the IT83XX_GCTRL_EPLR register in the
flash_it8xxx2_write_protection(false) is written as 1, the flash
write region will not be protected later, only be cleared by
power-on reset.

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>