Don't add prime tower area if single extrusion
Don't add prime tower area if single extrusion

Simple papercut fix.

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