Deleting a parent category doesn't properly update all child categories

[MtDewVirus]

When a parent category is deleted, the child categories are not all updated properly.

Steps to reproduce

1. Create a category titled Parent
2. Create a category titled Child1 with parent category set as Parent
3. Create a category titled Child2 with parent category set as Parent
4. Delete Parent category
5. Child1 gets updated properly in the database. It no longer has a category parent
6. Child2 still has the parent set to the deleted Parent category.

Tested with r8302

[hailin]

This is caused by the same bug in #7272.