As you now iOS 8 supports third party keyboards. But if you’ve installed and don’t want to use it anymore, you need to uninstall it. Deleting third party keyboards is similar to installing a new one. Removing this will obviously remove the ability to access the given keyboard until it has been downloaded and installed again.
Keep in mind that by deleting a keyboard like Swype, you also remove what it has learned about your typing habits and preferences, and thus it would have to relearn if you were to download the keyboard again. You can also delete a keyboards preference by removing the associated keyboard app that installs onto your iOS home screen.