gofmt: remove unneeded call to os.Exit

PrintDefaults already calls os.Exit(2).

Change-Id: I0d783a6476f42b6157853cdb34ba69618e3f3fcb
Reviewed-on: https://go-review.googlesource.com/24844Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/gofmt/gofmt.go

@@ -55,7 +55,6 @@ func report(err error) {
 func usage() {
 	fmt.Fprintf(os.Stderr, "usage: gofmt [flags] [path ...]\n")
 	flag.PrintDefaults()
-	os.Exit(2)
 }
 
 func initParserMode() {