Use recaptcha_ajax.js directly from Google

The script recaptcha_ajax.js contains minified non-free code which
we probably cannot redistribute. Since it talks to Google servers
anyway, it is better to just download it from Google directly instead
of shipping it.

This fixes #1932.

changelog.d/1932.bugfix

@@ -0,0 +1 @@
+Don’t ship recaptcha_ajax.js, use it directly from Google

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>