Don't reset the dataset type + plug for rm unused columns
This PR avoids resetting the dataset type when removing columns, and also introduces a filed in `TrainingArguments` to disable that behavior (in case the use wants to use some of those fields in an elaborate data collator).
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6683?src=pr&el=h1) Report
> Merging [#6683](https://codecov.io/gh/huggingface/transformers/pull/6683?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/1a779ad7ecb9e5215b6bd1cfa0153469d37e4274?el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `14.28%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6683/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6683?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6683      +/-   ##
==========================================
- Coverage   79.65%   79.60%   -0.05%     
==========================================
  Files         156      156              
  Lines       28250    28256       +6     
==========================================
- Hits        22503    22494       -9     
- Misses       5747     5762      +15     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6683?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/6683/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `55.63% <0.00%> (-0.53%)` | :arrow_down: |
| [src/transformers/training\_args.py](https://codecov.io/gh/huggingface/transformers/pull/6683/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmluZ19hcmdzLnB5) | `91.34% <100.00%> (+0.08%)` | :arrow_up: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6683/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `84.21% <0.00%> (-2.51%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6683?src=pr&el=footer). Last update [1a779ad...eea7e8d](https://codecov.io/gh/huggingface/transformers/pull/6683?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
