dt-bindings:iio:iio-binding.txt Drop file as content now in dt-schema

File contained generic IIO wide bindings.
Now part of the external dt-schema repository.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201031181242.742301-2-jic23@kernel.org