iio:adc: Drop false comment about lack of timestamp control

The timestamp control has been a function implemented in the core of
IIO for a long time, so this comment is incorrect and has clearly been
cut and paste into all these drivers.

The remainder of the comment added nothing and was confusing so dropped
that as well.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20210401145410.226917-1-jic23@kernel.org