A ringback tone is the tone you hear when you're making a call and the other party's phone is (supposed to be) ringing.
What you put into the phone is a ringtone - and it's normally in a folder named Ringtones. Use MyFiles to find the ringtone folder, select it (long-press it), then tap the trash can at the top and it'll be deleted. (Being there doesn't use much space, so you could just leave it there but not use it for the default ringtone or for any of your contacts' ringtones.)