In the Audio Window, under the small edit menu, you will find the "Select Unused" function.
Just select this, and the unused audio regions will become highlighted. Then go over to the Audio File menu (still from within the Audio Window), and select "Delete File(s)".
BE ADVISED... this erases those unused audio files from your hard drive, and is NOT undoable. Once it's done, it's done. If you merely want to remove the unused audio files from the song, but keep them on your hard drive, simply choose "Select Unused", and then hit the delete key.
Another option for removing unused audio files would be to invoke the "Save As Project" command, and store the song elsewhere on your drive, or to another hard drive altogether (for archiving, for instance). When you "save As Project", you have the option at the prompt of leaving unused audio behind, or deleting it all together, so it doesn't get copied to the new folder/location.
One thing though, make sure when you do choose "Select Unused", that you don't have tracks on your arrange page that are hidden. Logic considers these "in use", even though you can't visually see them.