iio: adc: ad7791: remove sample freq sysfs attributes

In the current state, these attributes are broken, because they are
registered already, and the kernel throws a warning.
The first registration happens via the `IIO_CHAN_INFO_SAMP_FREQ` flag from
the `ad_sigma_delta` driver.

In this commit these attrs are removed, and in the following the
IIO_CHAN_INFO_SAMP_FREQ behavior will be implemented, which replaces these
hooks.

This is done to make things a bit easier to review as there is a bit of
overlap in the patch if it's done all at once.

Fixes: a13e831f ("staging: iio: ad7192: implement IIO_CHAN_INFO_SAMP_FREQ")

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>