Recalculate disallowed borders when adding or removing objects

Because the new object may use an extruder that wasn't used so far, and if you remove an object it may remove the need for a certain extruder.

Contributes to issue CURA-2625.

cura/BuildVolume.py

@@ -104,9 +104,7 @@ class BuildVolume(SceneNode):
         root = Application.getInstance().getController().getScene().getRoot()
         new_number_of_objects = len([node for node in BreadthFirstIterator(root) if node.getMeshData() and type(node) is SceneNode])
         if new_number_of_objects != self._number_of_objects:
-            recalculate = False
-            if self._global_container_stack.getProperty("print_sequence", "value") == "one_at_a_time":
-                recalculate = (new_number_of_objects < 2 and self._number_of_objects > 1) or (new_number_of_objects > 1 and self._number_of_objects < 2)
+            recalculate = new_number_of_objects != self._number_of_objects
             self._number_of_objects = new_number_of_objects
             if recalculate:
                 self._onSettingPropertyChanged("print_sequence", "value")  # Create fake event, so right settings are triggered.