Remove theme registry fix that was needed for broken Drupal 6.0 - 6.6

Now that #258089: Themes cannot have a preprocess function without a corresponding .tpl.php file and #252430: Allow BASETHEME_ prefix in preprocessor function names have been added to Drupal core (as of Drupal 6.7), we should be able to safely remove the work-around I added in #249532: Allow subthemes to have preprocess hooks without tpl files..