fix: only ensure unprimed extruders are primed (CURA-3006)
fix: only ensure unprimed extruders are primed (CURA-3006)

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