Properly clean up printer connections that are no longer available.
Properly clean up printer connections that are no longer available.

Contributes to CURA-274

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