Remove unnecessary `del` in run_tf_glue.py example
Platform: Ubuntu 18.04 (Linux-4.15.0-1054-aws-x86_64-with-Ubuntu-18.04-bionic)
Python: 3.6.9
PyTorch: 1.4.0
TensorFlow: 2.0.0

Running `./examples/run_tf_glue.py` gives `KeyError: 'special_tokens_mask'`.

Diving into the code, it looks like there's an optional keyword argument in [`encode_plus()`](https://github.com/huggingface/transformers/blob/9d87eafd118739a4c121d69d7cff425264f01e1c/src/transformers/tokenization_utils.py#L834) named `return_special_tokens_mask` that defaults to False. I'm guessing that this argument was added recently and the example just needs to be updated?
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2670?src=pr&el=h1) Report
> Merging [#2670](https://codecov.io/gh/huggingface/transformers/pull/2670?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/9d87eafd118739a4c121d69d7cff425264f01e1c?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2670/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2670?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2670   +/-   ##
=======================================
  Coverage   74.51%   74.51%           
=======================================
  Files          87       87           
  Lines       14920    14920           
=======================================
  Hits        11117    11117           
  Misses       3803     3803
```



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<|||||>Indeed, thanks for catching that and removing it!