Improve delete handling code.

Change-Id: I3f09f00a2966ee246056ea11ec55d952da6aabc0
Drop blacklist number uniqueness constraint.

It's very hard to deal with that constraint in practise, so drop the
contraint from the DB and handle potential duplicate rows when querying.

Change-Id: Id28a415ba80cf100879b24de88b4b50638e900ca