bulk deletion of subclass objects not working

If I create a series of models roughly like this:

```
class A(PolymorphicModel):
some fields ...

class B(A):
# no extra fields... just new methods

class C(A):
# no extra fields... just new methods
....
```

then create some objects:

```
B().save()
C().save()
```

and then try and delete them:

```
A.objects.all().delete()
```

then I get an error something like "'C' object has no attribute 'b_ptr'"

Whereas if I delete them individually:

```
[i.delete() for i in A.objects.all()]
>[None, None]
```

then all is fine. I'm not sure if this is a bug or a feature, but it's frustrating because I'm going to have to override the admin to make deletion of inline objects work (I'm using B and C only to override methods on A, no additional fields are stored).

Start by reproducing the failure with the A, B, and C model examples and the A.objects.all().delete() call described in the issue. Inspect Django model-utils' polymorphic bulk-deletion path; done means deleting the subclass instances through A.objects.all().delete() succeeds without the b_ptr error, while individual deletion continues to work.

Written by the indexing model from the issue text.