jquery cannot hide span attribute in Drupal 7
Just a wild guess but

jQuery(document).ready(function($){
  $('span.fieldset-legend').hide();
  console.log('Hidden the spans');
}); 


Try the above code and see if "Hidden the spans" gets output on your javascript console. If it does, then the module you're using (Horizontal Groups? or whatever) is tampering with the spans and perhaps showing them after your code has run and hidden them.