Delete references to github.com/tensorflow/models/blob/master/tutorials

The directory is not maintained, and is no longer being used for tensorflow.org.

I am deleting it in tensorflow/models#8276

PiperOrigin-RevId: 302583413

Author: MarkDaoust

tensor2tensor/data_generators/ptb.py

@@ -49,8 +49,6 @@ def _build_vocab(filename, vocab_path, vocab_size):
   """Reads a file to build a vocabulary of `vocab_size` most common words.
 
    The vocabulary is sorted by occurrence count and has one word per line.
-   Originally from:
-   https://github.com/tensorflow/models/blob/master/tutorials/rnn/ptb/reader.py
 
   Args:
     filename: file to read list of words from.