doc: update install-source.html for SSA in Go 1.8

Fixes #17491

Change-Id: Ic070cbed60fa893fed568e8fac448b86cd3e0cbc
Reviewed-on: https://go-review.googlesource.com/31411Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

doc/install-source.html

@@ -43,17 +43,13 @@ architectures.
 	<code>amd64</code> (also known as <code>x86-64</code>)
 </dt>
 <dd>
-	A mature implementation. New in 1.7 is its SSA-based back end
-	that generates compact, efficient code.
+	A mature implementation.
 </dd>
 <dt>
 	<code>386</code> (<code>x86</code> or <code>x86-32</code>)
 </dt>
 <dd>
-	Comparable to the <code>amd64</code> port, but does
-	not yet use the SSA-based back end. It has an effective
-	optimizer (registerizer) and generates good code (although
-	<code>gccgo</code> can do noticeably better sometimes).
+	Comparable to the <code>amd64</code> port.
 </dd>
 <dt>
 	<code>arm</code> (<code>ARM</code>)