Don't save Document in pre_delete handler (fixes #2889)

FileField.delete() will, by default, save() the model instance it is
attached to. This is pointless if we're in the process of deleting the
Document -- and since Document.save() calls Document.update_matches(),
this scans all annotations every time a document is deleted.

pandora/document/models.py

@@ -302,7 +302,7 @@ def delete_document(sender, **kwargs):
         for f in glob('%s/*' % folder):
             if f != t.file.path:
                 os.unlink(f)
-        t.file.delete()
+        t.file.delete(save=False)
 pre_delete.connect(delete_document, sender=Document)
 
 class ItemProperties(models.Model):