LibJS: Don't use Handle<Value> for JS::Object private fields

There's no reason to use handles here, we can just mark private element
values from objects that store them.

Userland/Libraries/LibJS/Runtime/Object.h

@@ -37,7 +37,7 @@ struct PrivateElement {
 
     PrivateName key;
     Kind kind { Kind::Field };
-    Handle<Value> value;
+    Value value;
 };
 
 // Non-standard: This is information optionally returned by object property access functions.