iio: ad9361: Remove empty attribute group

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
iio: ad9361: Don't enable refclock manually

There is no need to manually enable the refclock, this will happen automatically
when any of the child clocks are enabled.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>