staging:iio:documentation: Drop most generic docs

These docs are both suffering from being out of date, and from being
superceeded by the documentation in Documentation/driver-api/iio

Note the inkern.txt drop is left for now as this is an area not
well covered by the more recent documentation outside staging.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20200905174721.216452-5-jic23@kernel.org