Roll-back of Issue #534594 by beejeebus, catch: This causes fatal errors from class loaders in certain modules (CTools, etc.)

update.php

@@ -376,10 +376,6 @@ if (empty($op) && update_access_allowed()) {
   // Set up theme system for the maintenance page.
   drupal_maintenance_theme();
 
-  // Rebuild the registry to ensure that removed hooks in modules do not result
-  // in undefined function errors and that newly defined hooks are called.
-  registry_rebuild();
-
   // Check the update requirements for Drupal.
   update_check_requirements();