As Brian said, deleting the presets you've created by using the preset manager is probably the easiest/cleanest way to remove the presets you have created.
Deleting the Form Builder directory from your site will not remove the presets that you have created. The purpose of having presets is to make it easier for you to create the form on different sites without having to recreate the form you've already made for one site. Therefore, the presets are not created and stored in any particular site.