Adding CC-100: Monolingual Datasets from Web Crawl Data (Datasets links are invalid)
## Describe the bug
The dataset links are no longer valid for CC-100. It seems that the website which was keeping these files are no longer accessible and therefore this dataset became unusable. 
Check out the dataset [homepage](http://data.statmt.org/cc-100/)  which isn't accessible.
Also the URLs for dataset file per language isn't accessible: http://data.statmt.org/cc-100/<language code here>.txt.xz (language codes: am, sr, ka, etc.)

## Steps to reproduce the bug
```python
from datasets import load_dataset
dataset = load_dataset("cc100", "ka")
```
It throws 503 error.

## Expected results
It should successfully download and load dataset but it throws an exception because the dataset files are no longer accessible.


## Environment info
Run from google colab. Just installed the library using pip:
```!pip install -U datasets```
