go spec: remove notes about gccgo limitations, now fixed

R=gri
CC=golang-dev
https://golang.org/cl/5164041

doc/go_spec.html

@@ -5265,7 +5265,5 @@ The following minimal alignment properties are guaranteed:
 <h2 id="Implementation_differences">Implementation differences - TODO</h2>
 <ul>
 	<li><code>len(a)</code> is only a constant if <code>a</code> is a (qualified) identifier denoting an array or pointer to an array.</li>
-	<li>In gccgo, <code>nil</code> maps are not treated like empty maps.</li>
-	<li>In gccgo, trying to send/receive from a <code>nil</code> channel causes a run-time panic.</li>
 </ul>
 </span>