iio: adc: ti-ads1015: Get rid of legacy platform data

Platform data is a legacy interface to supply device properties
to the driver. In this case we even don't have in-kernel users
for it. Just remove it for good.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>