obsolete code about deprecated sequences table in setUpCurrentUser()

Problem/Motivation

    // In many cases the anonymous user account is fine for testing purposes,
    // however, if we need to create a user with a non-empty ID, we need also
    // the "sequences" table.
    if (!\Drupal::moduleHandler()->moduleExists('system')) {
      $values['uid'] = 0;
    }

The sequences table is deprecated, so kernel tests will no longer be installing it.

This code is possibly wrong. The comment is definitely wrong.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet