doc: drop google +1 button from the home page

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5784048

doc/root.html

@@ -72,8 +72,6 @@ Linux, Mac OS X, Windows, and more.
 
 <div style="clear: both;"></div>
 
-<div id="plusoneRoot"><g:plusone annotation="none"></g:plusone></div>
-
 <script type="text/javascript" src="https://www.google.com/jsapi"></script>
 <script type="text/javascript" src="/doc/play/playground.js"></script>
 <script type="text/javascript">

doc/style.css

@@ -145,9 +145,6 @@ div#topbar .container {
 div#plusone {
 	float: right;
 }
-div#plusoneRoot {
-	float: right;
-}
 
 div#copyright {
 	color: #666;