MIPS: Loongson64: Remove unused loongson_reboot.

Commit 1bdb7b76 ("MIPS: Loongson64: Cleanup unused code")
left the loongson_reboot unused, delete it.

Signed-off-by: Jinyang He <hejinyang@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>