rtc: cleanup obsolete comment about struct rtc_class_ops

Commit ea369ea6 ("rtc: remove .open() and .release()") removes
open/release callback from struct rtc_class_ops.

Also commit 80d4bb51 ("RTC: Cleanup rtc_class_ops->irq_set_state")
and commit 696160fe ("RTC: Cleanup rtc_class_ops->irq_set_freq()")
removes irq callbacks.

So, just remove related comments so that readers will not be confused.

Signed-off-by: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200710080003.7986-1-misono.tomohiro@jp.fujitsu.com