dt-bindings:iio:temperature: Drop generic binding file.

So far, the thermocouple-type property described in here is only
used in a single driver.  Whilst I would like it to be more generally
used that hasn't happened yet and I don't see a reason to maintain
this small file in the hope that it happens.

I pushed for this generic binding in the first place.  Hopefully
we can bring it back at somepoint.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Patrick Havelange <patrick.havelange@essensium.com>
Link: https://lore.kernel.org/r/20201031184854.745828-47-jic23@kernel.org

Documentation/devicetree/bindings/iio/temperature/temperature-bindings.txt

-If the temperature sensor device can be configured to use some specific
-thermocouple type, you can use the defined types provided in the file
-"include/dt-bindings/iio/temperature/thermocouple.h".
-
-Property:
-thermocouple-type:	A single cell representing the type of the thermocouple
-			used by the device.