Fix remove columns from lazy dict
This was introduced in https://github.com/huggingface/datasets/pull/5252 and causing the transformers CI to break: https://app.circleci.com/pipelines/github/huggingface/transformers/53886/workflows/522faf2e-a053-454c-94f8-a617fde33393/jobs/648597

Basically this code should return a dataset with only one column:

```python
from datasets import *

ds = Dataset.from_dict({"a": range(5)})

def f(x):
    x["b"] = x["a"]
    return x

ds = ds.map(f, remove_columns=["a"])

assert ds.column_names == ["b"]
```