So you want a task that looks at your HTML, inspects all classes, ID's, and compare that against your CSS?
No, I haven't seen that.
Keep in mind as well, that if you are building a dynamic website, which I assume you are, using Roots and Wordpress, that there is relatively large possibility that you will end up using some Bootstrap CSS in the future, even if your site doesn't currently use it at this particular moment in time.
Trust me, everyone here is all about saving time, server queries, and cutting down on file size, but this seems like a little too much to me. With Roots and Grunt, your CSS file is already combined, concatenated, and minified. You may save a kilobyte or two by removing unused CSS, but it won't be much more than a kilobyte. That's 1024 characters.