staging: iio: ad2s1210: Use device-managed API

Use device-managed API to simplify the code.
The remove function is redundant now and can
be deleted.

Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>