rtc: cmos: remove cmos_rtc_ops_no_alarm

Clear RTC_FEATURE_ALARM to signal that alarms are not available instead of
having a supplementary struct rtc_class_ops with a NULL .set_alarm.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210110231752.1418816-5-alexandre.belloni@bootlin.com