This question has good answers on how to remove ringtones that one may have installed using iTunes on a Mac or PC.
However, it appears to be possible for iPhone users who do not have a Mac or PC to create and install custom ringtones using the iOS Garageband app (see here and here). Is it possible to remove those user-created custom ringtones from an iPhone (without having to find a Mac or PC with iTunes)? If so, how?