LibWeb: Unregister IntersectionObserver in finalize, not the destructor

Otherwise it UAFs the intersection root. Not sure how this didn't cause
a lot of crashes!

Userland/Libraries/LibWeb/IntersectionObserver/IntersectionObserver.h

@@ -66,6 +66,7 @@ private:
 
     virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(JS::Cell::Visitor&) override;
+    virtual void finalize() override;
 
     // https://www.w3.org/TR/intersection-observer/#dom-intersectionobserver-callback-slot
     JS::GCPtr<WebIDL::CallbackType> m_callback;