When the beta expires, or when we're ready to install the released version, how do we uninstall the beta? Do we simply trash the Adobe Audition.app bundle? Or do we need to run an uninstaller that cleans up files elsewhere on the computer? (I don't see an uninstaller on my system.)
There's no uninstaller, so you can just move the Adobe Audition.app to the trash.
If you want to fully clean everything, you can look in ~/Library/Preferences/Adobe/Audition/4.0 and delete anything you want from there.
There may be cruft (if you've crashed) from wherever you had your tempfolder (by default is here: ~/Library/Caches/TemporaryItems/Adobe/Audition/4.0)