LibWeb: Merge did_remove_attribute() into attribute_changed()

Instead of having two virtuals for attribute change notifications,
there is now only one. When the attribute is removed, the value is null.

Userland/Libraries/LibWeb/HTML/HTMLScriptElement.h

@@ -63,7 +63,6 @@ private:
     virtual void visit_edges(Cell::Visitor&) override;
 
     virtual void attribute_changed(DeprecatedFlyString const& name, DeprecatedString const& value) override;
-    virtual void did_remove_attribute(DeprecatedFlyString const&) override;
 
     // https://html.spec.whatwg.org/multipage/scripting.html#prepare-the-script-element
     void prepare_script();