Drupal 7 How to find Nodes, tagged with Broken Taxonomies?
Just as an FYI, even if the terms had been deleted properly, the references still would have been broken (see Database records not deleted for Term Reference Fields after Term is Deleted).

Either way, to answer your question, you could use use super useful Taxonomy Orphanage module.


  This module provides interfaces (drush, cron, admin form) for removing
  orphaned taxonomy term references from entities.