The Qt tools and libraries are usually installed in a directory tree rooted in a single directory. If you are on Windows, look at the Properties for the Qt Creator or Qt Assistant application icons on your Desktop. Go to the directory that contains those EXE files and then up the hierarchy until you reach the root Qt directory. You can delete that and Qt will be gone.

Of course, if you have used other directories for developing your projects, those won't be deleted along with Qt, but the Maintenance tool won't do that either.