Plant root hairs are very fine and take up disolved nutrients in solution from the soil. They can't take up toxins from other plants particularly dead ones. Imagine wild blackberries gowing next to deadly nightshade in a hedge, you can pick the blackberries as they are not poisonous whatsoever.
Some A4Aers put rhubarb leaves under their gooseberry bushes with no ill effects on the fruit. Likewise tomato and potato leaves are noxious but the tomatoes and potatoes delicious. Runner bean roots are poisonous but not the runner beans themselves. There are many similar examples.
So don't worry about planting anything near your old laburnum stump. Providing there is enough depth of soil for what you plant it will be perfectly ok.
