Lewis Elementary gives the definition of limito as
- to enclose within boundaries or limits, to bound
- to fix, settle, determine
and the definition of delimito as
- to mark out
I would suggest, given these definitions, that limito has the sense of establishing the existence and placement of a boundary ("You stay on your side of the back seat of the car and I'll stay on mine, and neither one of us will cross into the other's territory"), whereas delimito has the sense of creating a physical representation of that boundary ("Here, I'll put a piece of tape right down the middle of the seat so we can be sure not to cross the boundary").
I would say therefore that no, de in delimito is not actually redundant.