iio: Remove support for the legacy event config interface

Now that all drivers have been converted to the new event config interface we
can remove for the legacy event config interface. Also drop the '_new' suffix
for the event config interface callbacks, since those are the only callbacks
now.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>