Remove unused imports to MaterialManager

Contributes to issue CURA-6776.

cura/Machines/QualityManager.py

@@ -48,7 +48,6 @@ class QualityManager(QObject):
     def __init__(self, parent = None) -> None:
         super().__init__(parent)
         application = cura.CuraApplication.CuraApplication.getInstance()
-        self._material_manager = application.getMaterialManager()
         self._container_registry = application.getContainerRegistry()
 
         self._empty_quality_container = application.empty_quality_container

cura/Settings/ContainerManager.py

@@ -23,7 +23,6 @@ from UM.Settings.InstanceContainer import InstanceContainer
 
 import cura.CuraApplication
 from cura.Machines.ContainerTree import ContainerTree
-from cura.Machines.MaterialManager import MaterialManager
 
 if TYPE_CHECKING:
     from cura.CuraApplication import CuraApplication

cura/Settings/CuraStackBuilder.py

@@ -10,7 +10,6 @@ from UM.Settings.InstanceContainer import InstanceContainer
 
 from cura.Machines.ContainerTree import ContainerTree
 from cura.Machines.MachineNode import MachineNode
-from cura.Machines.MaterialManager import MaterialManager
 from .GlobalStack import GlobalStack
 from .ExtruderStack import ExtruderStack