rtc: remove rtc_time_to_tm and rtc_tm_to_time

There are no callers of the 32bit versions of rtc_time conversion
functions, drop them.

Link: https://lore.kernel.org/r/20200330201510.861217-1-alexandre.belloni@bootlin.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>