Remove useless placeholder text from JS functions

The README states...

> Don't add useless placeholder default texts like:

So I've removed the useless placeholder default text from the JS function snippets.

My main motivation for doing this was that removing the `argument` placeholder text with the delete key would break tabbing to the next field. Now that it been removed this is no longer an issue.

Cheers,
Louis

P.S. I've removed the `class_name` placeholder text from the `prototype` snippet. I'm not entirely sure this was the correct thing to do. What are your thoughts?