Delete old dangling logentry rows.

Old installs don't have the constraint setup on the content_type_id
attribute. It should look something like:

CONSTRAINT
`admin__content_type_id_ef0c694a2206d13_fk_django_content_type_id`
FOREIGN KEY (`content_type_id`) REFERENCES `django_content_type` (`id`)

As, a result, after the numerous migrations there may be log entries left
behine that refer to content_type's that don't exist any more.

This patch attempts to rectify this situation.

Fixes #253.

Change-Id: Ibbc4124ccfe8b71a54ce53219114444ffc90746a