LibWeb: Remove shadow roots from elements that are removed from the DOM

We currently create a shadow tree once for each DOM element that renders
with a shadow tree (e.g. <input>, <details>). If such an element is
removed from the DOM, we must remove its shadow tree. Otherwise, the
shadow tree will refer to the old document in perpetuity.

If the node is added back to a DOM, then recreate the shadow tree.

Userland/Libraries/LibWeb/HTML/HTMLInputElement.h

@@ -139,6 +139,7 @@ public:
     virtual void reset_algorithm() override;
 
     virtual void form_associated_element_was_inserted() override;
+    virtual void form_associated_element_was_removed(DOM::Node*) override;
 
     // ^HTMLElement
     // https://html.spec.whatwg.org/multipage/forms.html#category-label