Drop tasks for CAs that are no longer present

[timbru]

If a CA is removed, we may still have tasks on the queue for it - e.g. resync with parent etc.

Krill used to try to remove these tasks if a CA is removed, but this is a bit of a hassle. If the task is not removed properly then it will end up being rescheduled ad nauseam. The better solution here is to drop a task as 'done' when it's for a CA that has been removed.