Restore smart_holder_poc.h as-is on smart_holder branch (i.e. undo `P…

…YBIND11_SMART_HOLDER_PADDING`, which was meant for stress-testing only).

include/pybind11/detail/smart_holder_poc.h

@@ -138,31 +138,15 @@ inline bool is_std_default_delete(const std::type_info &rtti_deleter) {
            || rtti_deleter == typeid(std::default_delete<T const>);
 }
 
-// Meant to help detecting invalid `reinterpret_cast`s or similar.
-#ifdef PYBIND11_SMART_HOLDER_PADDING_ON
-#    define PYBIND11_SMART_HOLDER_PADDING(N) int PADDING_##N##_[11]
-#else
-#    define PYBIND11_SMART_HOLDER_PADDING(N)
-#endif
-
 struct smart_holder {
-    PYBIND11_SMART_HOLDER_PADDING(1);
     const std::type_info *rtti_uqp_del = nullptr;
-    PYBIND11_SMART_HOLDER_PADDING(2);
     std::shared_ptr<void> vptr;
-    PYBIND11_SMART_HOLDER_PADDING(3);
     bool vptr_is_using_noop_deleter : 1;
-    PYBIND11_SMART_HOLDER_PADDING(4);
     bool vptr_is_using_builtin_delete : 1;
-    PYBIND11_SMART_HOLDER_PADDING(5);
     bool vptr_is_external_shared_ptr : 1;
-    PYBIND11_SMART_HOLDER_PADDING(6);
     bool is_populated : 1;
-    PYBIND11_SMART_HOLDER_PADDING(7);
     bool is_disowned : 1;
-    PYBIND11_SMART_HOLDER_PADDING(8);
     bool pointee_depends_on_holder_owner : 1; // SMART_HOLDER_WIP: See PR #2839.
-    PYBIND11_SMART_HOLDER_PADDING(9);
 
     // Design choice: smart_holder is movable but not copyable.
     smart_holder(smart_holder &&) = default;