godoc: always use local jQuery script

Fixes #5075.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/10084044

lib/godoc/godoc.html

@@ -8,8 +8,7 @@
   <title>The Go Programming Language</title>
 {{end}}
 <link type="text/css" rel="stylesheet" href="/doc/style.css">
-<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.2/jquery.min.js"></script>
-<script type="text/javascript">window.jQuery || document.write(unescape("%3Cscript src='/doc/jquery.js' type='text/javascript'%3E%3C/script%3E"));</script>
+<script type="text/javascript" src="/doc/jquery.js"></script>
 {{if .Playground}}
 <script type="text/javascript" src="/doc/play/playground.js"></script>
 {{end}}