cmd/gofmt: remove impossible err!=nil conditional

cmd/gofmt: remove impossible err!=nil conditional

Change-Id: Ife63c766cefc14d2f6bdd3c706f913b1b6f99949
GitHub-Last-Rev: 04e23349e68a3ce12b7d33c0fd3ad8c7ffbd8aaa
GitHub-Pull-Request: golang/go#65929
Reviewed-on: https://go-review.googlesource.com/c/go/+/566099
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>