I'd hold onto it or just give it to a vassal. While their levies will never really be useful to you, their taxes will. Every little bit helps. If France ever fires up a dejure war against you to take it, you could always just instant surrender. Sure you'll lose some prestige but all you'll lose land-wise is that one county you don't really want anyway.
On the flip side, if france fires up a dejure war and you feel confident in your military, you could always duke it out with them...just for the sake of battle. Winning a defensive war can net you some healthy amounts of prestige and gold (from sieging down their provinces.)
The other thing you could do is stick an ambitious ruler in that county. Next time France has an independence revolt, you'd be surprised how effective some ambitious characters are at starting, and winning, their own wars to increase their...and thus your...holdings. (Provided you aren't running absolute crown authority)