net/http: eliminate defined-but-not-used var.

gc does not report this as an error, but go/types does.
(I suspect that constructing a closure counts as a reference
to &all in gc's implementation).

This is not a tool bug, since the spec doesn't require
implementations to implement this check, but it does
illustrate that dialect variations are always a nuisance.

LGTM=rsc, bradfitz
R=bradfitz
CC=golang-codereviews, gri, rsc
https://golang.org/cl/73850043

src/pkg/net/http/client_test.go

@@ -886,11 +886,9 @@ func TestClientTimeout(t *testing.T) {
 		t.Fatal("handler never got /slow request")
 	}
 
-	var all []byte
 	errc := make(chan error, 1)
 	go func() {
-		var err error
-		all, err = ioutil.ReadAll(res.Body)
+		_, err := ioutil.ReadAll(res.Body)
 		errc <- err
 		res.Body.Close()
 	}()