Remove all special handling of Google Code, which has shut down.

Commit 4ec2fd3e suggested that maybe the
shutdown warning should remain. However, it has been missing from Go 1.6
already, and by Go 1.7 people will most likely have realised that Google
Code has shut down.

Updates #10193.

Change-Id: I5749bbbe2fe3b07cff4edd20303bbedaeaa8d77b
Reviewed-on: https://go-review.googlesource.com/21189Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>