Is it possible to get rid of the part of the briefing that informs you of the mission objectives that you need to complete?
Bolded to point out that what some people have been suggesting is not what he's asking for.
What is it you actually want it hidden for?
I'm not sure, but this seems to me like the sort of situation where you're asking how to do something you don't really want anyway. Have you looked at the objectives screen, and particularly at what 'valid' versus 'invalid' objectives means, and what the validate-objective sexps (can't remember the actual name atm) do?