iio: dac: ad7303: remove platform data header

The information in the ad7303 platform_data header is unused, so it's dead
code.
This change removes it and it's inclusion from the driver.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20201001141004.53846-1-alexandru.ardelean@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>