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