wp: invalidate WpGlobal id when removed from registry

Once an object is removed from the registry, its id is invalidated and
can be later reused by other objects. WpGlobal objects may be
long-lived: e.g. those in tmp_globals may live over an add,remove,add
sequence for the same id, and a new WpGlobal must be used for the second
add. However, currently e.g. wp_registry_prepare_new_global may pick the
WpGlobal for a previously removed object from the tmp_globals list.

To address this, invalidate the WpGlobal id immediately when we get the
registry remove event.
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