Remove a default file template

Can anyone tell me how I can delete one of the default file templates? The PHPStorm template for an HTML file uses the .html extension - I like to use .htm, but I don't seem able to change it. I can create my own template easily enough, but it's confusing having the default one still in the "New..." context menu. The - button in the file template management dialogue is greyed out for the default templates.

Thanks!