Should I revoke my recommendation request for a particular letter? Is this impolite?
No, it would not be very polite. However, I do not understand why would you cancel the request. You can just have the letter and not use it. You are not obliged to send every single reference letter to the institution you apply.

If you feel like you must cancel the request, you could tell the professor that the institution you are applying requires three letters and you already have them. Do not forget to apologize that it is your mistake that you did not carefully read the instructions.