[dask] remove extra 'client' kwarg in DaskLGBMRegressor (#3906)

python-package/lightgbm/dask.py

@@ -710,7 +710,6 @@ class DaskLGBMRegressor(LGBMRegressor, _DaskLGBMModel):
         X: _DaskMatrixLike,
         y: _DaskCollection,
         sample_weight: Optional[_DaskCollection] = None,
-        client: Optional[Client] = None,
         **kwargs: Any
     ) -> "DaskLGBMRegressor":
         """Docstring is inherited from the lightgbm.LGBMRegressor.fit."""