This reverts commit f15915af.

CL 32851 fixed cmd/vet's handling of fmt.Formatter.

Updates #17057.

Change-Id: I3409100d16037645946fe7fe78fbb173e1648494
Reviewed-on: https://go-review.googlesource.com/33166
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>