cmd/stringer: remove os.Exit call from Usage func

The caller of Usage should call os.Exit -- Usage shouldn't call it.

Change-Id: I3decf662883fb2a6b19b7035138ee8a06a02de08
Reviewed-on: https://go-review.googlesource.com/7110
Reviewed-by: Andrew Gerrand <adg@golang.org>

cmd/stringer/stringer.go

@@ -94,7 +94,6 @@ func Usage() {
 	fmt.Fprintf(os.Stderr, "\thttp://godoc.org/golang.org/x/tools/cmd/stringer\n")
 	fmt.Fprintf(os.Stderr, "Flags:\n")
 	flag.PrintDefaults()
-	os.Exit(2)
 }
 
 func main() {