Ultimaker 2+ Connect doesn't support the exported material format

So we can't get people to export their materials. Hide the button.

resources/definitions/ultimaker2_plus_connect.def.json

@@ -22,8 +22,7 @@
             "0": "ultimaker2_plus_connect_extruder_0"
         },
         "supports_usb_connection": false,
-        "supports_network_connection": true,
-        "supports_material_export": true
+        "supports_network_connection": true
     },
 
     "overrides": {