cmd/internal/obj/loong64: remove invalid branch delay slots

cmd/internal/obj/loong64: remove invalid branch delay slots

Change-Id: I222717771019f7aefa547971b2d94ef4677a42c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/420979
Reviewed-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Run-TryBot: hopehook <hopehook@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: xiaodong liu <teaofmoli@gmail.com>