chore: Remove duckdb comment

Not planning to support this anymore, requires `fsspec` which isn't in `dev`

```
InvalidInputException
Traceback (most recent call last)
Cell In[6], line 5
       3 with duck._reader._opener.open(url) as f:
       4     fn = duck._reader._read_fn['.json']
----> 5     thing = fn(f.read())

InvalidInputException: Invalid Input Error: This operation could not be completed because required module 'fsspec' is not installed"
```

tools/datasets/_io.py

@@ -386,9 +386,6 @@ def get_backend(backend: _Backend, /) -> _Reader[Any, Any]:
         return _PandasReader("pandas")
     elif backend == "pyarrow":
         return _PyArrowReader("pyarrow")
-    elif backend == "duckdb":
-        msg = "Included in ``dev``, not investigated yet"
-        raise NotImplementedError(msg)
     elif backend in {"ibis", "cudf", "dask", "modin"}:
         msg = "Supported by ``narwhals``, not investigated yet"
         raise NotImplementedError(msg)