How to politely take back a request to be on an exam committee?
Be straightforward but tactful about your situation. You have a limited number of spaces on your committee and they are now filled, despite some initial confusion. This is just one of the challenges of assembling a committee and ensuring they're all in one place when they need to be. This additional professor will likely be understanding about that difficulty and will likely appreciate you letting them know about this change well in advance of any necessary committee meetings.