cmd/internal/obj/loong64: remove unused register alias definitions

cmd/internal/obj/loong64: remove unused register alias definitions

Change-Id: Ie788747372cd47cb3780e75b35750bb08bd166fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/542835
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Auto-Submit: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Michael Knyszek <mknyszek@google.com>