LibWeb: Move automatic browsing context creation to HTMLIFrameElement

We will soon have two DOM nodes which contain nested browsing contexts:
HTMLIFrameElement and HTMLObjectElement. Only HTMLIFrameElement should
have its nested context created automatically upon insertion, so move
the invocation of that logic to HTMLIFrameElement.

Userland/Libraries/LibWeb/HTML/HTMLIFrameElement.h

@@ -21,6 +21,7 @@ public:
 
 private:
     virtual void inserted() override;
+    virtual void removed_from(Node*) override;
     virtual void parse_attribute(const FlyString& name, const String& value) override;
 
     void load_src(const String&);