Cascading quereyset deletes don't go through treebeard and result in a corrupted tree.

[jxcl]

Steps to reproduce:

>>> from mysite.widgy_site import *
>>> from widgy.contrib.page_builder.models import Figure, Html
>>> f = Figure.add_root(site)
>>> h = f.add_child(site, Html)
>>> h.node.id
2
# This deletes the figure and django knows to cascade delete its node. However,
# it does not delete the node using treebeard's custom queryset that manages
# tree consistency. The SQL issued is just `DELETE FROM widgy_node WHERE id =
#1`, which leaves the Html's node orphaned
>>> Figure.objects.filter(pk=f.pk).delete()
>>> Node.find_problems()
# The Html node is orphaned.
([], [], [2], [], [])

Expected:

All deletes should use the treebeard queryset.

Observed:

Cascading querysets use the django builtin queryset and result in a corrupted tree.