Remove unused imports (#9235)

sklearn/__check_build/__init__.py

sklearn/cluster/tests/test_k_means.py

sklearn/cluster/tests/test_spectral.py

sklearn/cross_decomposition/__init__.py

sklearn/datasets/tests/test_base.py

sklearn/ensemble/tests/test_weight_boosting.py

sklearn/externals/joblib/my_exceptions.py

sklearn/feature_selection/tests/test_mutual_info.py

sklearn/linear_model/__init__.py

sklearn/linear_model/tests/test_coordinate_descent.py

sklearn/manifold/tests/test_spectral_embedding.py

sklearn/metrics/__init__.py

sklearn/metrics/tests/test_classification.py

sklearn/preprocessing/tests/test_function_transformer.py

sklearn/svm/bounds.py

sklearn/svm/tests/test_bounds.py

sklearn/tests/test_grid_search.py

sklearn/tests/test_init.py

sklearn/tree/tree.py

sklearn/utils/graph.py