iio: core: drop devm_iio_device_free() API call

It's unused so far, so it can be removed. Also makes sense to remove it
to discourage weird uses of this call during review.

This is the last user of 'devm_iio_device_match()', so it can be removed as
well in this patch.

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