Remove useless condition

The bug #880 has been resolved by handling stacking contexts stored as box
children, adding this condition.

But the stacking contexts stored as box children introduced other bugs. #1473
was another bug, and its fix also removed the problem of #880.

weasyprint/stacking.py

@@ -69,8 +69,6 @@ def from_box(cls, box, page, child_contexts=None):
         def dispatch(box):
             if isinstance(box, AbsolutePlaceholder):
                 box = box._box
-            elif isinstance(box, StackingContext):
-                box = box.box
             style = box.style
             absolute_and_z_index = (
                 style['position'] != 'static' and style['z_index'] != 'auto')