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

drivers/staging/iio/TODO

 2020-02-25
 
 
-ADI Drivers:
-CC the device-drivers-devel@blackfin.uclinux.org mailing list when
-e-mailing the normal IIO list (see below).
-
 Contact: Jonathan Cameron <jic23@kernel.org>.
 Mailing list: linux-iio@vger.kernel.org