rtc: mc13xxx: remove UIE signaling

The RTC core handles it since 6610e089 (RTC: Rework RTC code to use
timerqueue for events). So far, only the callbacks to the RTC core have
been removed, but not the handlers. Do this now.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>