iio: ABI: max31865: Drop in_filter_notch_centre_frequency as in main docs.

As this is the last element in the file, also delete the file.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Navin Sankar Velliangiri <navin@linumiz.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/20220626165511.602202-3-jic23@kernel.org