doc: remove mention of weekly tag from source install doc

R=golang-dev, dsymonds, bradfitz
CC=golang-dev
https://golang.org/cl/6442062

doc/install-source.html

@@ -247,8 +247,8 @@ Bugs can be reported using the <a href="http://code.google.com/p/go/issues/list"
 <h2 id="releases">Keeping up with releases</h2>
 
 <p>
-The Go project maintains two stable tags in its Mercurial repository:
-<code>release</code> and <code>weekly</code>.
+The Go project maintains a stable tag in its Mercurial repository:
+<code>release</code>.
 </p>
 
 <p>
@@ -269,12 +269,6 @@ $ hg update release
 $ ./all.bash
 </pre>
 
-<p>
-The <code>weekly</code> tag is updated about once a week, and should be used 
-only by those who are actively working on the Go core.
-To use the <code>weekly</code> tag run <code>hg update weekly</code> instead.
-</p>
-
 
 <h2 id="environment">Optional environment variables</h2>