os: do not use hosts file for windows tests

Not everyone has the file (http://golang.org/issue/4723#c4).

Update #4723.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7311091

src/pkg/os/os_test.go

@@ -41,7 +41,6 @@ var sysdir = func() (sd *sysDir) {
 		sd = &sysDir{
 			Getenv("SystemRoot") + "\\system32\\drivers\\etc",
 			[]string{
-				"hosts",
 				"networks",
 				"protocol",
 				"services",