LibJS: Move [[Fields]] to ECMAScriptFunctionObject

Userland/Libraries/LibJS/Runtime/FunctionObject.h

@@ -42,17 +42,6 @@ public:
     // This is for IsSimpleParameterList (static semantics)
     bool has_simple_parameter_list() const { return m_has_simple_parameter_list; }
 
-    // [[Fields]]
-    struct InstanceField {
-        StringOrSymbol name;
-        FunctionObject* initializer { nullptr };
-
-        void define_field(VM& vm, Object& receiver) const;
-    };
-
-    Vector<InstanceField> const& fields() const { return m_fields; }
-    void add_field(StringOrSymbol property_key, FunctionObject* initializer);
-
 protected:
     virtual void visit_edges(Visitor&) override;
 
@@ -66,7 +55,6 @@ private:
     Value m_bound_this;
     Vector<Value> m_bound_arguments;
     bool m_has_simple_parameter_list { false };
-    Vector<InstanceField> m_fields;
 };
 
 }