iio: adc: meson: remove unused timestamp channel

Remove IIO_CHAN_SOFT_TIMESTAMP channel because it's used only for
buffering mode which is not implemented.

Signed-off-by: George Stark <GNStark@sberdevices.ru>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230714114010.293440-2-gnstark@sberdevices.ru
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>