rtc: r9701: remove leftover comment

Commit 22652ba7 ("rtc: stop validating rtc_time in .read_time") removed
the code but not the associated comment.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201015191135.471249-1-alexandre.belloni@bootlin.com