staging: iio: cdc: ad7152: remove driver completely

The AD7153 part has been obsoleted for some time. The AD7152 part will be
obsolete in the coming future.
Moving it out of staging doesn't make sense anymore. Which makes the driver
enter a limbo state.

This patch removes the driver completely, so that no effort is placed on
it, allowing people to focus on other parts that will still be around.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>