MIPS: remove orphan sni_cpu_time_init() declaration

All uses of sni_cpu_time_init() were removed by
commit 4b550488 ("[MIPS] Deforest the function pointer jungle
in the time code."), so remove the orphan declaration, too.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>