user.autocomplete should not use database service

Nice! This is an easy one!

The property has been left inside. Remove this:

  /**
   * The database connection to query for the user names.
   *
   * @var \Drupal\Core\Database\Connection
   */
  protected $connection;

RTBC if turns green.

This looks great.

+++ b/core/modules/user/lib/Drupal/user/UserAutocomplete.php
@@ -49,13 +41,14 @@ class UserAutocomplete {
+   * @param \Drupal\Core\Entity\EntityManager $entity_manager
+   *   The entity manager.
...
+  public function __construct(ConfigFactoryInterface $config_factory, EntityManager $entity_manager, QueryFactory $entity_query) {

If we're fixing the constructor let's also change EntityManager to EntityManagerInterface.