doc: remove mentions of cloning from GitHub

Complete CL117178 removing all references to GitHub, and leaving
a small note to make sure we remember that it's not supported.

Change-Id: Id4257515a864875808fa7a67f002ed52cfd635a3
Reviewed-on: https://go-review.googlesource.com/130395
Reviewed-by: Daniel Martí <[email protected]>

Author: rasky

doc/contribute.html

@@ -405,7 +405,7 @@ <h3>Overview</h3>
 
 <ul>
 <li>
-<b>Step 1:</b> Clone the Go source code from go.googlesource.com
+<b>Step 1:</b> Clone the Go source code from <code>go.googlesource.com</code>
 and make sure it's stable by compiling and testing it once:
 <pre>
 $ git clone https://go.googlesource.com/go
@@ -469,12 +469,11 @@ <h3 id="checkout_go">Step 1: Clone the Go source code</h3>
 checked out from the correct repository.
 You can check out the Go source repo onto your local file system anywhere
 you want as long as it's outside your <code>GOPATH</code>.
-Either clone from
-<code>go.googlesource.com</code> or from GitHub:
+Clone from <code>go.googlesource.com</code> (not GitHub):
 </p>
 
 <pre>
-$ git clone https://github.com/golang/go   # or https://go.googlesource.com/go
+$ git clone https://go.googlesource.com/go
 $ cd go
 </pre>