Don't register registry at ContainerStack class any more

The fixture does this for us automatically now. It's not entirely neat since this is not cleaned up afterwards, but it does make sense to put it there in the fixture, logically.

Contributes to issue CURA-3427.

tests/Settings/TestCuraContainerRegistry.py

@@ -119,7 +119,6 @@ def test_addContainerBadSettingVersion(container_registry, definition_container)
 ])
 def test_loadTypes(filename, output_class, container_registry):
     #Mock some dependencies.
-    UM.Settings.ContainerStack.setContainerRegistry(container_registry)
     Resources.getAllResourcesOfType = unittest.mock.MagicMock(return_value = [os.path.join(os.path.dirname(os.path.abspath(__file__)), "stacks", filename)]) #Return just this tested file.
 
     def findContainers(container_type = 0, id = None):