JS can't be executed with a keyboard shortcut (only through the menus using keyboard accelerators). Also, you'll need to better explain what you want to achieve.
I think you're not quite getting how bookmarks work. They are not "attached" to a page, per se. They can point to a page, but they can also do other things entirely, or nothing at all. And there can be multiple bookmarks pointing to the same page. So the idea of an "unused" bookmark is not really a correct one. You can probably do what you want using a script, but you'll need to define a good way of identifying the bookmarks you want to delete.