Clean-up migrate executable

Problem/Motivation

Follow-up from #2503999-32: Large volume entity migrations run out of memory
Core maintainer comment https://www.drupal.org/node/2503999#comment-10266631

+++ b/core/modules/migrate/src/MigrateExecutable.php
@@ -568,7 +568,17 @@ protected function attemptMemoryReclaim() {
+    // Entity storage can blow up with caches so clear them out.
+    $container = \Drupal::getContainer();
+    /** @var \Drupal\Core\Entity\EntityManagerInterface $manager */
+    $manager = $container->get('entity.manager');

There's \Drupal::entityManager() for that

Proposed resolution

Use \Drupal::entityManager()

Remaining tasks

User interface changes

API changes

Data model changes