netutil: Ditch go1.3 build tag.

We only support Go 1.4+ nowadays; Go 1.2 and earlier are ancient.

Change-Id: I128998550afbf287ad797bb9df905b31af0ff952
Reviewed-on: https://go-review.googlesource.com/17791
Run-TryBot: David Symonds <dsymonds@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

netutil/listen_test.go

@@ -2,11 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build go1.3
-
-// (We only run this test on Go 1.3 because the HTTP client timeout behavior
-// was bad in previous releases, causing occasional deadlocks.)
-
 package netutil
 
 import (