Use recaptcha_ajax.js directly from Google

This was originally done in commit c75b71a397,
but got reverted on this branch due to the PR (#3677) being based on the wrong
branch.

We're ready to merge this to master now, so let's make it match
release-v0.33.3.

synapse/static/client/register/index.html

@@ -4,7 +4,7 @@
 <meta name='viewport' content='width=device-width, initial-scale=1, user-scalable=no, minimum-scale=1.0, maximum-scale=1.0'> 
 <link rel="stylesheet" href="style.css">
 <script src="js/jquery-2.1.3.min.js"></script>
-<script src="js/recaptcha_ajax.js"></script>
+<script src="https://www.google.com/recaptcha/api/js/recaptcha_ajax.js"></script>
 <script src="register_config.js"></script>
 <script src="js/register.js"></script>
 </head>