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

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

This reverts commit 6c9b7c9df190a456af7f7249d409347a6d0070fc.

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>