iio: dac ad8801: drop driver remove function

iio: dac ad8801: drop driver remove function

Remove driver remove callback in the ad8801 driver.

By making use of devm_iio_device_register(), we no longer need a driver
remove callback. Also since this was the last user of spi_get_drvdata(),
we can drop the call to spi_set_drvdata().

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241120-iio-regulator-cleanup-round-6-v1-7-d5a5360f7ec3@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>