cmd/internal/obj/loong64: remove case 17 in func asmout

cmd/internal/obj/loong64: remove case 17 in func asmout

There is no relative optab item case 17, remove it.

Change-Id: I3ceaa3283c3641afafd46362737ff847a1d80665
Reviewed-on: https://go-review.googlesource.com/c/go/+/565617
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: abner chenc <chenguoqi@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Qiqi Huang <huangqiqi@loongson.cn>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
Reviewed-by: Michael Knyszek <mknyszek@google.com>