staging:iio: remove orphaned function definition.

I have no idea what this was, or what it did, but it doesn't
do it any more.
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/iio/ring_generic.h

@@ -211,8 +211,6 @@ int iio_ring_buffer_register_ex(struct iio_dev *indio_dev, int id,
 				const struct iio_chan_spec *channels,
 				int num_channels);
 
-void iio_ring_access_release(struct device *dev);
-
 /**
  * iio_ring_buffer_unregister() - unregister the buffer from IIO core
  * @indio_dev: the device with the buffer to be unregistered