hook_civicrm_managed leaves stale entity when last declaration disappears

When an extension declares an entity via hook_civicrm_managed, it's inserted. When the declaration is removed from the hook, the entity should also be removed from the hook. This happens correctly if there are still some other (unrelated) entities declared by the hook; but if a module doesn't have any entities at all, then the cleanup doesn't work correctly.

Discovered while working on CRM-11736.