Comments are a bit of a sticky situation, really.
They're regularly used incorrectly for a variety of reasons. If you see a comment that doesn't actually do what comments should - request more information, ask for clarification, or make suggestions for improvement - those comments are fair game to flag for a moderator to remove.
This is true anywhere on the network.
Now, whether the moderator agrees with your flag will largely depend on the site and their local comment policy. Some sites are pretty liberal with their comments and delete few of them. Some sites, like one of the ones I moderate, are very active in deleting pretty much any comment that doesn't do one of the things I've already mentioned.
What response you'll get from a moderator will depend on the site, so it's probably a good idea to take that into account. You can always research specific sites' comment removal policy. Many have them. If they don't, you can always ask. Some sites have chat rooms where the moderators are active... that might be a good place to reach out and see what they think, otherwise, there's the child meta of the site (this is the big network-wide meta).
So, which flag to use?
In a general case like this where the statement is more benign than attack-y, I'd go with "No longer needed". That's definitely true and hopefully a moderator will agree.
If the comment veers into insulting the supposed professor "Your professor is an idiot and should be fired for teaching you this way"... that's rude/abusive and it should be flagged as such.
If your flag is declined, you have a couple of options
- you can flag again but with a custom reason explaining your flag and asking that the comment be removed. The generic flags sometimes don't tell us much so we have to do a bit of digging - or not - so a hint is often useful when we're hip-deep in flags.
- ask about it in chat. Even if there aren't any moderators there, you're likely going to find some experienced users who can recommend a course of action.
- ask about it on meta. I'm guessing that asking about comments not being deleted on Stack Overflow might be a bit of overkill, particularly with how many active moderators they have, but on smaller sites, meta is the right place to go when you're looking for help understanding why your flags are declined.
Please do make an effort to phrase the question as a request for understanding/information rather than being accusatory towards the moderators. It makes us more likely to respond and explain.