iio: ad_sigma_delta: Remove no-op trigger ops

iio: ad_sigma_delta: Remove no-op trigger ops

The IIO core handles a trigger ops with all NULL callbacks the
same as if the trigger ops itself was NULL.

Remove the empty trigger ops from the interrupt trigger driver to slightly
reduce the boilerplate code. Object size of the driver module is also
slightly reduced.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20211031142130.20791-3-lars@metafoo.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>