How are comments removed when they are no longer relevant?

I see that a comment can be flagged but I take that as a function for comments that are offensive in some way. In cases where a comment is not relevant because the question/answer has morphed in to something else, what is done to weed out comments that are no longer useful?

As example: Crime Investigation in a fantasy world

Flag the comment as Obsolete (that's an option under the flag) or if all the comments on a post are obsolete, flag the post for moderator attention. We'll clean it up.