Don't compute changeset for entities that are going to be deleted

This is somewhat related to

http://www.doctrine-project.org/jira/browse/DDC-2761

I'm not sure if it fixes that particular problem, but it certainly helps with some unrelated issues I'm seeing in my code. My theory is that doctrine doesn't even need to compute changesets when an entity is scheduled to be removed, because all that would accomplish is to insert an UPDATE statement immediately before the DELETE statement for the entity. 

The test suite passes, but I have not added a test specifically for this change (mostly because I wouldn't know exactly what to test). I could try to create one if necessary, though