runtime: remove dead code

LGTM=bradfitz, dave, ruiu
R=rsc, iant, bradfitz, dave, ruiu
CC=golang-codereviews
https://golang.org/cl/116610043

src/pkg/runtime/sys_linux_amd64.s

@@ -221,7 +221,6 @@ TEXT runtime·sigreturn(SB),NOSPLIT,$0
 
 TEXT runtime·mmap(SB),NOSPLIT,$0
 	MOVQ	8(SP), DI
-	MOVQ	$0, SI
 	MOVQ	16(SP), SI
 	MOVL	24(SP), DX
 	MOVL	28(SP), R10