IndirectContainers don't delete proxies unless insertedContentRelation is ore:proxyFor

[escowles]

IndirectContainers can use any predicate to link to the URI they are proxies for, but ore:proxyFor is hard-coded:

https://github.com/projecthydra/active_fedora/blob/master/lib/active_fedora/associations/contained_finder.rb#L43

Proxies can be created and read correctly, but they cannot be deleted.