In PrestaShop 1.6.x we can use theme configurator module available in your dashboard. So, with brand new responsive theme it is no longer necessary to alter any file. You have to follow instructions below:

Log in to shop dashboard and then open  "preferences > themes" exactly as i show on image: preferences > themes

On the right side  click on "theme configurator" button: Theme configurator button (It is also available from "modules > modules" section)

On theme configurator module page, scroll page down and at the end of the first form you will find "display subcategories" option. Just switch its value to NO