undo shuffle change

this is not going to help with current tf.data semantics. so removing it.

official/transformer/v2/data_pipeline.py

@@ -236,9 +236,6 @@ def _read_and_batch_from_files(
   # Remove examples where the input or target length exceeds the maximum length,
   dataset = dataset.filter(lambda x, y: _filter_max_length((x, y), max_length))
 
-  if shuffle:
-    dataset = dataset.shuffle(20000)
-   
   if static_batch:
     dataset = dataset.padded_batch(
         batch_size // max_length, ([max_length], [max_length]),