How do I remove the site name from the head title in drupal 8?
Until metatag's module port has not finished you should code a bit for doing that.

If I were you I'll edit YOURTHEME.theme file and add this:

/**
 * Implements hook_preprocess_HOOK() for HTML document templates.
 *
 * Adds body classes if certain regions have content.
 */
function YOURTHEME_preprocess_html(&amp;$variables) {
    unset ($variables['head_title']['name']);
  }