Do not unset class properties because it causes un-initialization

Problem/Motivation

See: https://3v4l.org/V8uuP

The first occurrence of this problem were indefinited in #3110266-46: Support serialization of private properties → here: https://github.com/drupal/core/blob/10.1.5/lib/Drupal/Core/Config/Config...

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet