go_spec: no known implementation differences anymore

R=r, rsc, iant, ken
CC=golang-dev
https://golang.org/cl/5781048

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of March 1, 2012",
+	"Subtitle": "Version of March 7, 2012",
 	"Path": "/ref/spec"
 }-->
 
@@ -5499,9 +5499,3 @@ The following minimal alignment properties are guaranteed:
 <p>
 A struct or array type has size zero if it contains no fields (or elements, respectively) that have a size greater than zero. Two distinct zero-size variables may have the same address in memory.
 </p>
-
-
-<h2 id="Implementation_differences">Implementation differences - TODO</h2>
-<ul>
-	<li><span class="alert"><code>len(x)</code> is only a constant if <code>x</code> is a (qualified) identifier denoting an array or pointer to an array.</span></li>
-</ul>