Features of IterableDataset set to None by remove column
### Describe the bug

The `remove_column` method of the IterableDataset sets the dataset features to None.

### Steps to reproduce the bug

```python
from datasets import Audio, load_dataset

# load LS in streaming mode
dataset = load_dataset("librispeech_asr", "clean", split="validation", streaming=True)

# check original features
print("Original features: ", dataset.features.keys())

# define features to remove: we KEEP audio and text
COLUMNS_TO_REMOVE = ['chapter_id', 'speaker_id', 'file', 'id']

dataset = dataset.remove_columns(COLUMNS_TO_REMOVE)

# check processed features, uh-oh!
print("Processed features: ", dataset.features)

# streaming the first audio sample still works
print("First sample:", next(iter(ds)))
```

**Print Output:**

```
Original features:  dict_keys(['file', 'audio', 'text', 'speaker_id', 'chapter_id', 'id'])
Processed features:  None
First sample: {'audio': {'path': '2277-149896-0000.flac', 'array': array([ 0.00186157,  0.0005188 ,  0.00024414, ..., -0.00097656,
       -0.00109863, -0.00146484]), 'sampling_rate': 16000}, 'text': "HE WAS IN A FEVERED STATE OF MIND OWING TO THE BLIGHT HIS WIFE'S ACTION THREATENED TO CAST UPON HIS ENTIRE FUTURE"}
```

### Expected behavior

The features should be those **not** removed by the `remove_column` method, i.e. audio and text.

### Environment info

- `datasets` version: 2.7.1
- Platform: Linux-5.10.133+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.7.15
- PyArrow version: 9.0.0
- Pandas version: 1.3.5

(Running on Google Colab for a blog post: https://colab.research.google.com/drive/1ySCQREPZEl4msLfxb79pYYOWjUZhkr9y#scrollTo=8pRDGiVmH2ml)

cc @polinaeterna @lhoestq 