staging: iio: cdc: ad7746: Drop unused i2c_set_clientdata()

As the comment states, this was only used in remove() and now
there is no explicit remove() function to make use of it.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220626122938.582107-10-jic23@kernel.org