registry: fix issues with dangling WpGlobal objects causing assertion failures

The main problem observed is when a link that is owned by a WpLink
is removed from the server because one of the linked nodes is gone.
This would cause the APPEARS_ON_REGISTRY flag to go away but the
WpGlobal would still remain in the globals list...

To fix this, forcibly remove the global from the globals list when
it is removed from the registry, even if it is still owned by some
proxy. The proxy at that point is unable to function anyway, because
we make sure to destroy the pw_proxy by removing FEATURE_BOUND when
the global is removed from the registry.

Additionally, ref global->proxy before removing FEATURE_BOUND to
prevent crashing. If the proxy owns the global and the pw-proxy-destroyed
signal causes whoever owns the proxy to drop his reference, _deactivate()
will crash because no-one will be holding a ref to the proxy.