Hide in the source code instead of in the separate whitelist.
Removes the only printf false positive in the standard library.

Change-Id: I99285e67588c7c93bd56d59ee768a03be7c301e7
Reviewed-on: https://go-review.googlesource.com/74590
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>