Now that vet loads from source,
fmt can always be correctly resolved,
so the fmt.Formatter type is always available,
so we can reinstate the check.

Change-Id: I17f0c7fccf6960c9415de8774b15123135d57be8
Reviewed-on: https://go-review.googlesource.com/37692
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>