staging:iio:light: drop stale ABI docs

There are no remaining light drivers in staging/iio.
The content of this file are either included in the non staging
ABI docs, or don't seem to be used in any current driver.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20200905174721.216452-4-jic23@kernel.org