Rufus has a mode to restore a bootable USB to generic storage media (because, of course, if an application allows you to change something, it usually also allows you to revert those changes).
See that Boot selection menu in Rufus? Just select Non bootable in there with whatever file system you want, and Rufus will happily restore your USB to a single partition non-bootable regular media, that will use all the available capacity.
Next time, you may also want to look at the Rufus FAQ, which has a dedicated entry for this.