Remove superfluous set to nullptr
Remove superfluous set to nullptr

Shared pointers are already set to nullptr by default.

Contributes to issue CURA-1210.

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