Remove unused import

This was no longer needed since ExtruderManager is now a singleton. It's no longer in CuraApplication.

Contributes to issues CURA-1278 and CURA-351.

cura/CuraApplication.py

@@ -31,7 +31,6 @@ from UM.Settings.ContainerRegistry import ContainerRegistry
 
 from UM.i18n import i18nCatalog
 
-from . import ExtruderManager
 from . import PlatformPhysics
 from . import BuildVolume
 from . import CameraAnimation