Add helpers to remove leftover field storage config

Example of usage in an example.install:

/**
 * Remove ghostly remains of old 'resources' content type (renamed resource).
 *
 * Implements hook_update_N().
 */
function example_update_9002() {
  \Drupal::service('module_installer')->install(['migration_helpers']);
  \Drupal::service('migration_helpers.exorcise_ghost_config_entity')->removeBundleFromFieldStorageConfig('node', 'resources');
  \Drupal::service('module_installer')->uninstall(['migration_helpers']);
}                           

Really need to stress that none of this code checks if what it is removing is safe to remove. It will totally remove a completely still-in-use set of config. That it is truly no longer of use is an exercise for the user of this code.

Two additional methods on ExorciseGhostConfigEntity:

1. removeFieldStorageConfig($entity_type, $bundle, $field_name)
2. cleanUpEmptyEntityType($entity_type)

The latter is called by removeBundleFromFieldStorageConfig($entity_type, $bundle) automatically.