\Drupal\migrate\Plugin\MigrateDestinationInterface::fields() $migration argument is deprecated

Problem/Motivation

The $migration argument of \Drupal\migrate\Plugin\MigrateDestinationInterface::fields is

   * @param \Drupal\migrate\Plugin\MigrationInterface $migration
   *   Unused, will be removed before Drupal 9.0.x. Defaults to NULL.

This was added in #2543568: Remove the md_entity destination plugin hack.

Proposed resolution

Work out what to do.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet