Clean nsgaii_with_init_trials code

[hrntsm]

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Motivation

Response to #165 (comment) .

Description of the changes

Since sharing the code with Optuna itself is likely to improve maintainability in the future, I updated to inherit the code from NSGAII.

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[hrntsm]

@nabenabe0928
Thank you for your comment. I have created a PR that reflects it.

[y0z]

LGTM