Enable shap sparse test. (#6332)

Author: trivialfis

tests/python-gpu/test_gpu_prediction.py

@@ -212,10 +212,6 @@ def test_shap(self, num_rounds, dataset, param):
            tm.dataset_strategy, shap_parameter_strategy)
     @settings(deadline=None, max_examples=20)
     def test_shap_interactions(self, num_rounds, dataset, param):
-        if dataset.name == 'sparse':
-            issue = 'https://github.com/dmlc/xgboost/issues/6074'
-            pytest.xfail(reason=f'GPU shap with sparse is flaky: {issue}')
-
         param.update({"predictor": "gpu_predictor", "gpu_id": 0})
         param = dataset.set_params(param)
         dmat = dataset.get_dmat()