LibWeb: Make FormAssociatedElement inherit from HTMLElement

The new event target implementation requires us to downcast an
EventTarget to a FormAssociatedElement to check if the current Element
EventTarget has a form owner to setup a with scope for the form owner.

This also makes all form associated elements inherit from
FormAssociatedElement where it was previously missing.

https://html.spec.whatwg.org/#form-associated-element

Userland/Libraries/LibWeb/HTML/HTMLInputElement.h

@@ -11,9 +11,7 @@
 
 namespace Web::HTML {
 
-class HTMLInputElement final
-    : public HTMLElement
-    , public FormAssociatedElement {
+class HTMLInputElement final : public FormAssociatedElement {
 public:
     using WrapperType = Bindings::HTMLInputElementWrapper;
 
@@ -43,9 +41,6 @@ private:
     virtual void inserted() override;
     virtual void removed_from(Node*) override;
 
-    // ^HTML::FormAssociatedElement
-    virtual HTMLElement& form_associated_element_to_html_element() override { return *this; }
-
     // ^DOM::EventTarget
     virtual void did_receive_focus() override;