Remove _static folder in doc directory

TrivialFix:
_static folder and the inside files are auto generated
when running docs, _ga is unused, so remove these.

Change-Id: I43b8cff826a98329607eed65a75eaf1ebaf54646

doc/source/_ga/layout.html

@@ -1,17 +0,0 @@
-{% extends "!layout.html" %}
-
-{% block footer %}
-{{ super() }}
-<script type="text/javascript">
-var gaJsHost = (("https:" == document.location.protocol) ? "https://ssl." : "http://www.");
-document.write(unescape("%3Cscript src='" + gaJsHost + "google-analytics.com/ga.js' type='text/javascript'%3E%3C/script%3E"));
-</script>
-<script type="text/javascript">
-try {
-var pageTracker = _gat._getTracker("UA-17511903-3");
-pageTracker._setDomainName("none");
-pageTracker._setAllowLinker(true);
-pageTracker._trackPageview();
-} catch(err) {}</script>
-{% endblock %}
-