Previously, we only removed links from children to dead parents.
This patch is semantically the same as Funman's earlier
vlc_object_detach_unlocked() patch. It is merely faster and less clean.

Unfortunately, this means it is impossible to detect leaked children.
That being noted, leaving dead parents "semi-attached" in objects tree
is not thread-safe anyhow. Basically, we either automatically detach
objects (and miss leaks), or require children to be detached/destroyed
before their parent is destroyed (and get leaks detection, but more
coding pain).