Well, the problem with the former (knowledge harbored within the brain) is that it's very vague and hard to define.
If I have, say, a method to improve the efficacy of VX (an easily weaponizable nerve toxin). As a utilitarian I conclude this information is going to be harmful, I can purge it of my hard-drive, I can burn the papers I used to come up with this... etc.
But I can't wipe my head clean of the information, at best I can resign to never talk about it to anyone and to not accord it much import, such that I may forget it. But that's not destruction per-say, it's closer to lying, not sharing the information with anyone (even if asked specifically), or to biasing your brain towards transmitting and remembering certain pieces of information (which we do all the time).
However I don't see anything contentious with this case, nor with any other case of information-destruction, as long as it is for the greater utility.
I think in general people don't advocate for destroying/forgetting information because:
a) It's hard to do
b) As a general rule of thumb the accumulation of information seems to be a good thing, even if the utility of a specific piece of information is not obvious
But this is more of a heuristic, an exact principle.