staging: iio: remove mention of defunct list

The ADI device-drivers-devel list no longer exists.

Cc: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Link: https://lore.kernel.org/r/69b3e45e7666a1dd74a83df0b84ef8a63bf090ea.1614082343.git.baruch@tkos.co.il


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>