staging:iio:adis16220: Remove adis16220 driver

The ADIS16220 part has been obsoleted, which makes it hard to get the
hardware to even test the driver. Considering this there is no expectation
that the driver will be cleaned up and be able to move out of staging, so
remove the driver.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>