staging:iio:adc:ad7280a: Drop unused timestamp channel.

The driver doesn't support buffered mode, so a timestamp channel that
is entirely hidden from userspace without buffer mode is rather pointless.
Drop it.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://lore.kernel.org/r/20220206190328.333093-9-jic23@kernel.org