Remove dead code from ContentAccess::addFieldValues()

Problem/Motivation

This code part is not used anymore anywhere in the code:

    static $anonymous_user;

    if (!isset($anonymous_user)) {
      // Load the anonymous user.
      $anonymous_user = new AnonymousUserSession();
    }

Let's get rid of that.

It was used to be used back in the old days: https://git.drupalcode.org/project/search_api/-/commit/dc21f12ca665a81bc...

Steps to reproduce

Proposed resolution

Remaining tasks