fix(smart_holder): Use std::default_delete. (#4924)

Ensures `std::default_delete<T>` is used to look up the deleter for a type instead of `delete` directly.

include/pybind11/detail/smart_holder_poc.h

@@ -103,7 +103,7 @@ struct guarded_delete {
 
 template <typename T, typename std::enable_if<std::is_destructible<T>::value, int>::type = 0>
 inline void builtin_delete_if_destructible(void *raw_ptr) {
-    delete static_cast<T *>(raw_ptr);
+    std::default_delete<T>{}(static_cast<T *>(raw_ptr));
 }
 
 template <typename T, typename std::enable_if<!std::is_destructible<T>::value, int>::type = 0>