Problem/Motivation
While going through drupal_common_theme() in #1885560: [meta] Convert everything in theme.inc to Twig, a definition for theme_help() was found. theme_help() was removed during Drupal 7 development at #240873-47: Move custom help settings to blocks.
theme_help() API reference page for Drupal 6, notice no links for D7 or D8:
http://api.drupal.org/api/drupal/includes%21theme.inc/function/theme_help/6
Proposed resolution
Remove these lines from drupal_common_theme().
Remaining tasks
Small patch needs to be written to remove these lines from drupal_common_theme().
User interface changes
n/a
API changes
n/a