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/130395Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

doc/contribute.html

@@ -405,7 +405,7 @@ This is an overview of the overall process:
 
 <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 @@ In addition to a recent Go installation, you need to have a local copy of the so
 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>