Disabling mirror feature while mirror is being updated does not decrement capacity size before disabling

Scenario

1. Set project as mirror
2. Mirror gets scheduled to run, by being added to a Redis set
3. Mirror gets disabled
4. Mirror does not get removed from the Redis set, meaning it will stay there forever

This problem affects the number of mirrors we can schedule every time since that project, even though it is not a mirror anymore, still has its key in the Redis set

\cc @DouweM