Don't allow passing persisted objects to -[RLMRealm addObject:]

This didn't work correctly if the object was persisted in a different
Realm (it just created an empty row rather than copying it), and
`createInRealm:withObject` is a much better way to support copying
objects between Realms as it doesn't involve a method doing something
very different from what it normally does.
copy in RLMValidatedObjectForProperty