iio: buffer: remove iio_buffer_set_attrs() helper

The iio_buffer_set_attrs() is no longer used in the drivers, so it can be
removed now.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20200929125949.69934-10-alexandru.ardelean@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>