LibWeb: Remove declarations for non-existent methods

Author: https://github.com/AtkinsSJ
Commit: 7891ad2a28
Pull-request: https://github.com/SerenityOS/serenity/pull/17203
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibWeb/DOM/Node.h

@@ -313,8 +313,6 @@ public:
 
     bool is_following(Node const&) const;
 
-    void prepend_child(JS::NonnullGCPtr<Node> node);
-
     Node* next_in_pre_order()
     {
         if (first_child())