LoongArch: Clean up loongson3_smp_ops declaration

Since loongson3_smp_ops is not used in LoongArch anymore, let's remove
it for cleanup.

Fixes: f2ac457a ("LoongArch: Add CPU definition headers")
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/include/asm/loongson.h

@@ -14,8 +14,6 @@
 #include <asm/addrspace.h>
 #include <asm/bootinfo.h>
 
-extern const struct plat_smp_ops loongson3_smp_ops;
-
 #define LOONGSON_REG(x) \
 	(*(volatile u32 *)((char *)TO_UNCACHE(LOONGSON_REG_BASE) + (x)))