Remove unnecessary code to suppress warnings

[koic]

Rails cops has been removed from RuboCop 0.72.
https://github.com/rubocop-hq/rubocop/releases/tag/v0.72.0

As a result of verification, the code to guard a warning is not necessary already.

Related PR ... rubocop/rubocop-rails#83

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• Added tests.
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