test_tf_common: remove un_used mixin class parameters

Fixes #6590 

@sshleifer Is that all for this issue? are there any other cleaning left which I did not understand ❗  

Thanks
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6866?src=pr&el=h1) Report
> Merging [#6866](https://codecov.io/gh/huggingface/transformers/pull/6866?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/59a6a32a61a87f9a1cccb57c3b4df725384d34ae?el=desc) will **decrease** coverage by `0.24%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6866/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6866?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6866      +/-   ##
==========================================
- Coverage   79.91%   79.67%   -0.25%     
==========================================
  Files         157      157              
  Lines       28795    28795              
==========================================
- Hits        23012    22942      -70     
- Misses       5783     5853      +70     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6866?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/6866/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdHJhbnNmb194bC5weQ==) | `20.53% <0.00%> (-21.21%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6866/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `83.95% <0.00%> (-2.26%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6866/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `82.41% <0.00%> (-0.26%)` | :arrow_down: |
| [src/transformers/data/data\_collator.py](https://codecov.io/gh/huggingface/transformers/pull/6866/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFfY29sbGF0b3IucHk=) | `92.30% <0.00%> (+0.40%)` | :arrow_up: |
| [src/transformers/tokenization\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/6866/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fcmVmb3JtZXIucHk=) | `95.00% <0.00%> (+13.33%)` | :arrow_up: |
| [src/transformers/tokenization\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/6866/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxuZXQucHk=) | `90.09% <0.00%> (+23.42%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6866?src=pr&el=footer). Last update [59a6a32...b4f1cad](https://codecov.io/gh/huggingface/transformers/pull/6866?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>The next step is to look for other places those class attributes are defined and remove them:


```bash
$ git grep test_pruning | grep tf
tests/test_modeling_tf_distilbert.py:    test_pruning = True
tests/test_modeling_tf_longformer.py:    test_pruning = False  # pruning is not supported
tests/test_modeling_tf_transfo_xl.py:    test_pruning = False
tests/test_modeling_tf_xlnet.py:    test_pruning = False
```
```bash
$ git grep test_torchscript | grep tf
tests/test_modeling_tf_common.py:    test_torchscript = True
tests/test_modeling_tf_distilbert.py:    test_torchscript = True
tests/test_modeling_tf_longformer.py:    test_torchscript = False
tests/test_modeling_tf_transfo_xl.py:    test_torchscript = False
```
<|||||>Thanks for the support.
I also found `test_head_masking` which was unused. So deleted it too. Let me know if you didn't want that to happen.

```bash
$ git grep -e "test_head" | grep tf
tests/test_modeling_tf_distilbert.py:    test_head_masking = True
tests/test_modeling_tf_longformer.py:    test_headmasking = False  # head masking is not supported
```

Thanks

PS: suggestion for any other issue, which I can pick up would be great. I am looking under label `help wanted`, etc