iio: adc: Remove ad400x

The driver for the AD4000 series of devices has changed a lot throughout
the upstreaming review process. As a result, the driver in ADI Linux is
very different from the upstream one.
Remove the ad400x driver to replace it by upstream-like ad4000 driver in a
following commit.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>

Author: machschmitt