[dask] remove unused private _client attribute

[StrikerRUS]

Refer to #3883 (comment).

[jameslamb]

Why is _client not required? The other properties on scikit-learn estimators seem to all have an attribute like that with a single leading underscore:

LightGBM/python-package/lightgbm/sklearn.py

Lines 320 to 325 in c324e89

	self._class_weight = None
	self._class_map = None
	self._n_features = None
	self._n_features_in = None
	self._classes = None
	self._n_classes = None

[StrikerRUS]

The other properties on scikit-learn estimators seem to all have an attribute like that with a single leading underscore:

Yeah, but this is not required by sklearn. It is made for storing underlying value of a property (with training _). You chose to use function for this purpose.

[jameslamb]

ok. I thought that #3883 (comment) referred to literally that one line you commented on, not the property completely.

[github-actions]

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