Remove task templates if required features are removed during `Dataset.map`
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This PR fixes a bug reported by @craffel where removing a dataset's columns during `Dataset.map` triggered a `KeyError` because the `TextClassification` template tried to access the removed columns during `DatasetInfo.__post_init__`:

```python
from datasets import load_dataset

# `yelp_polarity` comes with a `TextClassification` template
ds = load_dataset("yelp_polarity", split="test")
ds
# Dataset({
#     features: ['text', 'label'],
#     num_rows: 38000
# })

# Triggers KeyError: 'label' - oh noes!
ds.map(lambda x: {"inputs": 0}, remove_columns=ds.column_names)
```

I wrote a unit test to make sure I could reproduce the error and then patched a fix.
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