net/http: enable TestTransportRemovesDeadIdleConnections on Plan 9

This issue has been fixed in CL 31390.

Fixes #15464.

Change-Id: I35e088f37bf3b544100ff131c72690bcfd788e5b
Reviewed-on: https://go-review.googlesource.com/31393
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/transport_test.go

@@ -441,9 +441,6 @@ func TestTransportMaxPerHostIdleConns(t *testing.T) {
 }
 
 func TestTransportRemovesDeadIdleConnections(t *testing.T) {
-	if runtime.GOOS == "plan9" {
-		t.Skip("skipping test; see https://golang.org/issue/15464")
-	}
 	defer afterTest(t)
 	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
 		io.WriteString(w, r.RemoteAddr)