cmd/vet: replace hasMethod with isFormatter

cmd/vet: replace hasMethod with isFormatter

The former checks if a type has a method called "Format". The latter
checks if a type satisfies fmt.Formatter.

isFormatter does exactly what we want, so it's both simpler and more
accurate. Remove the only use of hasMethod in its favor.

Change-Id: Idc156a99081c3308f98512b87011a04aa8c6638d
Reviewed-on: https://go-review.googlesource.com/91215
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Rob Pike <r@golang.org>