staging: adis16255 delete driver

This patch deletes the adis16255 driver from staging as a similar
implementation exists inside the iio subsystem.

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>