How to politely take back a request to be on an exam committee? I need to have three faculty members on my Qualifying Exam committee, with one non-adjunct faculty of my department to be chair. On our department guidelines webpage, the 'non-adjunct' bit was not specified. 

I had therefore emailed three professors, two from another departent (CSE), and one adjunct from mine (EE). One of the CSE professors agreed, the EE professor agreed but told me he couldn't be chair because he was adjunct. I therefore got another EE professor to be chair. Now I will have to email the other CSE professor to tell him I don't need him to be on the committee anymore since I've got people already. This wouldn't have happened had I known about the rule for chair. 

How do I email him in the politest way possible? He's currently out of country so meeting in person isn't possible.
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.