mips: remove unused function mc146818_set_rtc_mmss

I have checked carefully: this function is unused, so remove it.

The last caller appears to have been removed in 2007 in
commit 4b550488 ("[MIPS] Deforest the function pointer jungle in the time code.")

mc146818-time.h is included only in three files:
- arch/mips/mti-malta/malta-time.c
- arch/mips/loongson64/numa.c
- arch/mips/loongson2ef/common/time.c

Also, remove unused macros USEC_AFTER/USEC_BEFORE.

Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>