iio: dac: ltc2632: remove some unused defines

These two defines are unused since the driver was introduced in commit
02b829f9 ("iio: dac: Add support for ltc2632 DACs").

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>