Make convert-grammars a little quieter

No need to say "Done" at the end. We're obviously done; the script is
exiting.

script/convert-grammars

@@ -298,5 +298,3 @@ if $options[:output].is_a?(IO)
 else
   File.write($options[:output], YAML.dump(yaml))
 end
-
-$stderr.puts("Done")