MIPS: SGI-IP27: Remove not used definition TICK_SIZE in ip27-timer.c

After commit f5ff0a28 ("[MIPS] Use generic NTP code for all MIPS
platforms"), TICK_SIZE is not used in ip27-timer.c for many years,
remove it.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>