my_custom_migration.install

<?php

/**
 * Implements hook_uninstall().
 */
function my_custom_migration_uninstall() {
  $configs  = [
    'migrate_plus.migration_group.my_custom_migration',
    'migrate_plus.migration.company',
    'migrate_plus.migration.contact',
  ];
  foreach($configs as $config) {
    $delete = \Drupal::database()->delete('config')
      ->condition('name', $config)
      ->execute();
  }
  drupal_flush_all_caches();
}