Don't set extruder_nr per object for single extrusion printers
Don't set extruder_nr per object for single extrusion printers

CURA-1754

ynotstartups/Wanhao,Curahelper/Cura,totalretribution/Cura,fieldOfView/Cura,hmflash/Cura,senttech/Cura,totalretribution/Cura,Curahelper/Cura,ynotstartups/Wanhao,senttech/Cura,fieldOfView/Cura,hmflash/Cura