That is the briefing.html, which is fine. It defines that there is an objective, but it will always start out shown (i.e. "ACTIVE"). Create a file called "init.sqf" in the same directory as mission.sqm and briefing.html and put in it:
"1" objStatus "HIDDEN";
so that the objective you've defined will be hidden on mission start (init.sqf, or init.sqs if that is present, is run at the start of the mission). If you really don't want to create a new file, just put the code in the player object's init line or something...