runtime: remove unnecessary noescape to fix netbsd

I introduced this build failure in golang.org/cl/9302 but failed to
notice due to the other failures on the dashboard.

Change-Id: I84bf00f664ba572c1ca722e0136d8a2cf21613ca
Reviewed-on: https://go-review.googlesource.com/9363Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

src/runtime/os_netbsd.go

@@ -15,7 +15,6 @@ func sigaction(sig int32, new, old *sigactiont)
 //go:noescape
 func sigaltstack(new, old *sigaltstackt)
 
-//go:noescape
 func sigfwd(fn uintptr, sig uint32, info *siginfo, ctx unsafe.Pointer) {
 	throw("sigfwd not implemented")
 }