staging:iio:adis16260: Remove support for orientation mapping

The adis16260 driver implements a unique feature in that it allows to change the
orientation of the gyroscope channel by specifying the orientation in platform
data. This feature is as far as I can see unused though and makes the driver
unnecessarily complex. So this patch removes the support for it. If it turns
out we need this, the cleanest approach to implement it is by adding support for
orientation mapping inside the IIO core so it is available to all drivers.

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