There's a workaround but it may take some time depending on how many songs you have. Here's what I did:
Acquire a CD-RW
Make a new playlist called convert
Drag a number of protected songs to the convert playlist to total <80 min.
Burn playlist to CD-RW
Once burned, import back to iTunes as MP3.
DRM will be removed when you burn the audio disc allowing you to rip it back as unprotected MP3.
Not the sexiest but it works. There are programs around there that purport to allow you to convert your DRM music but they either cost, don't work, or both...
***Edit to add: Found this software that might work for you. It has a free version. I've never used it so YMMV. It skips the whole burn to CD rip back and just "burns" the playlist to a virtual CD drive.
DVD neXt COPY iTurns FREE