undo CL 10017045 / 6ec8c49421a0

It was never tested and also breaks Windows.

run.go doesn't yet support the proper !windows,!plan9 syntax.

««« original CL description
test: do not run SIGCHLD test on Plan 9

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/10017045
»»»

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/10024044

test/sigchld.go

-// +build !windows !plan9
+// +build !windows
 // cmpout
 
 // Copyright 2009 The Go Authors. All rights reserved.