iio: adc: max1363: Drop provision to provide an IIO channel map via platform data

Back in the days of board files, platform data was used to provide
information on the mapping from ADC channel to an analog signal
from another device. We've long since moved to doing this via
device tree. Hence drop the support from the max1363 driver which is
the only driver still providing this.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20220821161058.2207185-1-jic23@kernel.org