iio:ABI docs: Drop device specific docs for _powerdown for the adf4371

The interface is standard and the extra info provided on exactly what is
being powered down is not of interest to most users.  Hence this doesn't
need it's own documentation and can rely on the docs in sysfs-bus-iio

Fixes
$ scripts/get_abi.pl validate
Warning: /sys/bus/iio/devices/iio:deviceX/out_altvoltageY_powerdown is defined 2 times:  ./Documentation/ABI/testing/sysfs-bus-iio-frequency-adf4371:36  ./Documentation/ABI/testing/sysfs-bus-iio:588

Cc: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reported-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20210117153816.696693-4-jic23@kernel.org