Remove a warning message of `PedAnovaImportanceEvaluator` #388

Job	Run time
test (git+https://github.com/optuna/optuna.git)	3m 30s
test (git+https://github.com/optuna/optuna.git)	3m 17s
test (git+https://github.com/optuna/optuna.git)	3m 23s
	10m 10s

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