Chances are, they already know. However; if you wanted to bring it up, that's not a problem. Remember however, it's NOT WHAT you say, but HOW you say it.
Don't use my words below as WHAT to say or NOT. I'm simply showing an example of how you can bring up the fact that you received an LOA in a positive or in a negative way.
i.e.
When questioned: "Why should we give you a nomination"; (Or something similar)
BAD: Well, I know I'm really a good candidate because the academy sent me an LOA. And that means they want me because they know I'm good. Not everyone gets an LOA, so that means I must be better than most of the others.
GOOD: I've always wanted to serve my country in the military; especially as a officer. I've been working very hard to get the best grades I can and to prove to myself that I had what it takes to receive such an honor as an appointment to the academy. Receiving an LOA from the Air Force academy was in itself such an honor and makes me realize that I'm on the right path and makes me want the air force academy that much more.