fix deleting apps that have content ratings (bug 945466)

Was trying to set IARC storefront data (which fetches the app) after the app was deleted. No likey.

```
DoesNotExist: Addon matching query does not exist.

Stacktrace (most recent call last):

  File "django/core/handlers/base.py", line 111, in get_response
    response = callback(request, *callback_args, **callback_kwargs)
  File "newrelic/hooks/framework_django.py", line 485, in wrapper
    return wrapped(*args, **kwargs)
  File "amo/decorators.py", line 157, in wrapper
    return f(*args, **kw)
  File "amo/decorators.py", line 149, in wrapper
    return f(*args, **kw)
  File "addons/decorators.py", line 33, in wrapper
    return f(request, addon, *args, **kw)
  File "amo/decorators.py", line 32, in wrapper
    return func(request, *args, **kw)
  File "mkt/developers/decorators.py", line 49, in wrapper
    return fun()
  File "mkt/developers/decorators.py", line 28, in <lambda>
    *args, **kw)
  File "amo/decorators.py", line 51, in wrapper
    return f(request, *args, **kw)
  File "mkt/developers/views.py", line 158, in delete
    addon.delete(msg='Removed via devhub', reason=reason)
  File "django/db/transaction.py", line 224, in inner
    return func(*args, **kwargs)
  File "addons/models.py", line 453, in delete
    self.set_iarc_storefront_data(disable=True)
  File "mkt/webapps/models.py", line 1172, in set_iarc_storefront_data
    delocalized_self = Addon.objects.get(pk=self.pk)
  File "django/db/models/manager.py", line 131, in get
    return self.get_query_set().get(*args, **kwargs)
  File "django/db/models/query.py", line 366, in get
    % self.model._meta.object_name)
```