AthenaServices: Work around conditions dependency issues.

Some conditions objects have pointers to parts of other conditions objects, which violates the lifetime guarantees of conditions containers (a pointer you get from a conditions container is guaranteed to be valid until the end of the current event, but not past that). In some cases, this can be dealt with by replacing the pointers with CondLink, but that is sometimes rather inconvenient. Try to work around this for now by ensuring that when we delete an object, we also try to clean the containers of other objects that may depend on it.