[utils] Remove duplicated line

This line was added by accident in 42393ce234

youtube_dl/utils.py

@@ -761,7 +761,6 @@ def unified_strdate(date_str):
         '%Y-%m-%dT%H:%M:%S.%fZ',
         '%Y-%m-%dT%H:%M:%S.%f0Z',
         '%Y-%m-%dT%H:%M:%S',
-        '%Y-%m-%dT%H:%M:%S',
     ]
     for expression in format_expressions:
         try: