Remove settings from SmartStackMixin

As properties now take validation settings from the final class, we no
longer need to redefine settings here, so I've removed them.

src/openflexure_microscope_server/things/scan_workflows.py

@@ -329,8 +329,6 @@ class SmartStackCompatibleSettings(Protocol):
 class SmartStackMixin:
     """A mixin for scan workflows that use smart stacking."""
 
-    _class_settings = {"validate_properties_on_set": True}
-
     stack_images_to_save: int = lt.setting(default=1, ge=1, le=9)
     """The number of images to save in a stack.