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>

Author: abner-chenc

src/cmd/internal/obj/loong64/a.out.go

@@ -225,8 +225,6 @@ const (
 	REGZERO = REG_R0 // set to zero
 	REGLINK = REG_R1
 	REGSP   = REG_R3
-	REGRET  = REG_R20 // not use
-	REGARG  = -1      // -1 disables passing the first argument in register
 	REGRT1  = REG_R20 // reserved for runtime, duffzero and duffcopy
 	REGRT2  = REG_R21 // reserved for runtime, duffcopy
 	REGCTXT = REG_R29 // context for closures