Are 'active' and 'staging' required in config_directories? Is it OK to delete the 'active' and 'staging' entries automatically created in settings.php? I don't plan on using them, but it's not clear to me whether some process might expect them to exist.
The default.settings.php file contains the following comment:


  The staging directory is used for configuration imports; the active directory is not used by default, since the default storage for active configuration is the database rather than the file system (this can be changed; see "Active configuration settings" below).


In short:


The staging directory is used only when importing configuration from the UI
The active directory is not used by default from Drupal 8, which saves the configuration data in the database, like Drupal 7 did with the persistent variables


In both the cases, I would expect Drupal to correctly handle the case the global $config_directories is not set.