MIPS: remove orphan sb1250_time_init() declaration

All uses of sb1250_time_init() have been removed by
commit d527eef5

 ("[MIPS] Sibyte: Finish conversion to modern
time APIs."), so remove the orphan declaration.

The comments about sb1250_time_init() is useless in arch_init_irq(),
so remove these comments.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>