Here is what happened in this case:
The questions is clearly off-topic for the site and was closed by Dave. But two answers were posted before the closing.
The first one was a one line answer by a new unregistered anonymous user (i.e. didn't have the reputation to comment):
To be NP-complete, it must be in NP.
So I converted it to a comment. But the second answer was similar in essence (by a regular relatively high rep user) and I felt it would be unfair to just convert one of them, so I converted the second one. Then I felt that it would better if I just delete them since they were not adding any information not on the Wikipedia page.
I personally didn't feel good about it after deleting because we don't have a clear policy on this (and the second answerer knew that the questions is clearly off-topic and that we don't answer them on cstheory), my justification for deletion was previous meta discussions we had where IIRC the conclusion was that we should not answer these questions even in the comments and should just forward the OP to the place where he/she could find the answer (Wikipedia, textbook, ...).
I also felt that deleting them would be in line with closing clearly off-topic questions by moderators (i.e. inside minimal mod intervention). I would have suggested adopting a policy like:
answers not containing new information (in addition to what is on Wikipedia, ...) to clearly off-topic questions should be deleted, preferably by OP.
But I think we probably don't need to do this for the following reason:
The feedback from Math.SE guys on adding Math.SE to the off-topic flagging wizard has been quite positive, so I think it we can simply migrate these questions with their answers to Math.SE and there is no need to delete the answers by us if the question is migrated to Math.SE.
ps: I also made a small edit to the general FAQ (Math.SE entry) based on the feedback from Math.SE folks.