iio: core: remove iio_priv_to_dev() helper

All users of this helper have been updated to not use it.
Remove it now, so that we don't need to move it when creating the
iio_dev_opaque structure.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>