Remove superfluous `require`s from RuboCop specs [RUN AS-IF-FOSS]

What does this MR do?

We do require 'rubocop' already in spec/support/rspec.rb.

This came up during a review of omnibus-gitlab!4898 (comment 522227314).

Does this MR meet the acceptance criteria?

Conformity

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  • I have not included a changelog entry because it's a developer-facing change.
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