alpha: rtc: remove unused set_mmss ops

The .set_mmss and .setmmss64 ops are only called when the RTC is not
providing an implementation for the .set_time callback.

On alpha, .set_time is provided so .set_mmss64 is never called. Remove the
unused code.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>