When a question is migrated, it ends up being locked on the origin site. This is good so that the question doesn't completely disappear, confusing the asker, but a side effect of being locked is that the asker can't delete the question on their own. It's also impossible to flag a locked post and request for it to be deleted. Deleting the stub is not without precedent - Jeff deleted these yesterday:
The fact that not all the sites were launched at the same time has caused some questions that I've asked on some sites to be more appropriate on other sites (once they existed), and since the primary reason for the stub is for the asker's benefit, I request that the author of a migrated question be allowed to delete it when they feel it's no longer necessary to keep around.