staging:iio:gyro:adis16260 remove some unused headers

These should have gone with the iio_chan_spec conversion patch.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>