net/http: don't pile up defers in b.N loop

One defer was not removed in CL61150043.

LGTM=dvyukov
R=bradfitz, dvyukov
CC=golang-codereviews
https://golang.org/cl/64600044

src/pkg/net/http/serve_test.go

@@ -2258,7 +2258,6 @@ func BenchmarkClientServer(b *testing.B) {
 		if err != nil {
 			b.Fatal("Get:", err)
 		}
-		defer res.Body.Close()
 		all, err := ioutil.ReadAll(res.Body)
 		res.Body.Close()
 		if err != nil {