An easy way I've found is to go to the Finder, right-click or control-click on the UFO and select "Show Package Contents". You will see a file called
groups.plist. Open that in a plain text editor (like BBEdit or TextMate) and then search for
"'" (the apostrophe) and replace with
"" (nothing). Save the file and reopen the UFO in RoboFont. If the UFO was already open in RoboFont, it will put up a message about the UFO having been changed outside of RoboFont, which you can confirm.
(Edit: Those should be "dumb" quotes above, not smart quotes. The forum software seems to be "fixing" them automatically.)