Remove unnecessary difference computation
Remove unnecessary difference computation

The roof is already subtracted from supportAreas at this point, so it's not in the bottom anyway.

Contributes to issue CURA-3491.

ROBO3D/CuraEngine,Ultimaker/CuraEngine,ROBO3D/CuraEngine,Ultimaker/CuraEngine,alephobjects/CuraEngine,ROBO3D/CuraEngine,alephobjects/CuraEngine,alephobjects/CuraEngine