Revert "iio: temperature: maxim_thermocouple: add MAX31856 part"

This reverts commit 535fba29.

Seems the submitter (er me, hang head in shame) didn't look at the datasheet
enough to see that the registers are quite different.

This needs to be reverted because a) would never work b) to open it  be added
to a Maxim RTDs (Resistance Temperature Detectors) under development by author

Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>