runtime: remove spurious register loads for openbsd/amd64 kqueue

The kqueue system call takes no arguments, hence there should be no need
to zero the registers used for the first syscall arguments.

Change-Id: Ia79b2d4f4d568bb6795cb885e1464cf1fc2bf7c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/174128
Run-TryBot: Benny Siegert <bsiegert@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>

src/runtime/sys_openbsd_amd64.s

@@ -348,9 +348,6 @@ TEXT runtime·sysctl(SB),NOSPLIT,$0
 
 // int32 runtime·kqueue(void);
 TEXT runtime·kqueue(SB),NOSPLIT,$0
-	MOVQ	$0, DI
-	MOVQ	$0, SI
-	MOVQ	$0, DX
 	MOVL	$269, AX
 	SYSCALL
 	JCC	2(PC)