Deleting an object should remove the SolrDocument of the proxy

[jcoyne]

https://github.com/projecthydra-labs/activefedora-aggregation/blob/master/lib/active_fedora/aggregation/association.rb#L24

If the target object is deleted, I believe that Fedora removes the proxy object automatically. But ActiveFedora doesn't know this happened, so the proxies solr document was never removed. Thus members_ids.size can return the old count (say 5) instead of the accurate count that members.size would return (4).