Move cache clearing in FieldConfig::postDelete() to FieldConfigBase

Problem/Motivation

Config fields are split between two classes:

- FieldConfigBase, in the Field core component
- FieldConfig, which inherits from it, in the field module.

FieldConfigBase handles clearing caches on save, in postSave():

    // Clear the cache.
    \Drupal::service('entity_field.manager')->clearCachedFieldDefinitions();

However, for deleting a field, it's in FieldConfig::postDelete():

    // Clear the cache upfront, to refresh the results of getBundles().
    \Drupal::service('entity_field.manager')->clearCachedFieldDefinitions();

    // Notify the entity storage.
    foreach ($fields as $field) {
      if (!$field->deleted) {
        \Drupal::service('field_definition.listener')->onFieldDefinitionDelete($field);
      }
    }

This part of the code in postDelete() should be moved up to the parent class for consistency.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes