I'm trying to remove the Armory research requirement for the Valkyrie but nothing happens when I click on remove item.
None.
It appears impossible to remove the requirement once you've added it (or a preexisting one) - I've run into this myself. Guess the only thing you can do is replacing it with "Starport" - it will be superfluous but at least it won't get in the way.
Who am I, and why should I care?
You need to click on the Buttons Sets tab before clicking on the Dat Requirements tab, because firegraft is a functional piece of software. Doing this should fix your issue.