doc: PORTING: drop flash_area_read_is_empty()

uint8_t flash_area_align(const struct flash_area *);

/*< What is value is read from erased flash bytes. */

uint8_t flash_area_erased_val(const struct flash_area *);

/*< Reads len bytes from off, and checks if the read data is erased. Returns

    1 if empty (that is containing erased value), 0 if not-empty, and -1 on

    failure. */

int     flash_area_read_is_empty(const struct flash_area *fa, uint32_t off,

                     void *dst, uint32_t len);

/*< Given flash area ID, return info about sectors within the area. */

int     flash_area_get_sectors(int fa_id, uint32_t *count,

                     struct flash_sector *sectors);