staging: iio: Drop ADIS16060 driver from staging

staging: iio: Drop ADIS16060 driver from staging

This part has been declared "not for new designs". It is now
difficult to obtain and we have had no-one come forward with hardware
making it difficult to proceed with the necessary work to move this
driver out of staging.

The device uses two separate chip selects and would require locking
between them which is thought to be difficult to enforce without
non trivial changes in the SPI subsystem.  This work simply isn't worth
doing given the status of the part and the fact no one seems to have
gone for a similar hardware design since this one.

If anyone does have access to one of these and is willing to contribute
the time necessary then we can reevaluate dropping the driver.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>