FORCE_REDOWNLOAD does not work
## Describe the bug
With GenerateMode.FORCE_REDOWNLOAD, the documentation says 
    +------------------------------------+-----------+---------+
    |                                    | Downloads | Dataset |
    +====================================+===========+=========+
    | `REUSE_DATASET_IF_EXISTS` (default)| Reuse     | Reuse   |
    +------------------------------------+-----------+---------+
    | `REUSE_CACHE_IF_EXISTS`            | Reuse     | Fresh   |
    +------------------------------------+-----------+---------+
    | `FORCE_REDOWNLOAD`                 | Fresh     | Fresh   |
    +------------------------------------+-----------+---------+

However, the old dataset is loaded even when FORCE_REDOWNLOAD is chosen.

## Steps to reproduce the bug
```python

import pandas as pd
from datasets import load_dataset, GenerateMode
pd.DataFrame(range(5), columns=['numbers']).to_csv('/tmp/test.tsv.gz', index=False)
ee = load_dataset('csv', data_files=['/tmp/test.tsv.gz'], delimiter='\t', split='train', download_mode=GenerateMode.FORCE_REDOWNLOAD)
print(ee)
pd.DataFrame(range(10), columns=['numerals']).to_csv('/tmp/test.tsv.gz', index=False)
ee = load_dataset('csv', data_files=['/tmp/test.tsv.gz'], delimiter='\t', split='train', download_mode=GenerateMode.FORCE_REDOWNLOAD)
print(ee)

```

## Expected results
Dataset({
    features: ['numbers'],
    num_rows: 5
})
Dataset({
    features: ['numerals'],
    num_rows: 10
})

## Actual results
Dataset({
    features: ['numbers'],
    num_rows: 5
})
Dataset({
    features: ['numbers'],
    num_rows: 5
})


## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.8.0
- Platform: Linux-4.14.181-108.257.amzn1.x86_64-x86_64-with-glibc2.10
- Python version: 3.7.10
- PyArrow version: 3.0.0
