Remove SafeMode module

[koic]

Follow up #7341

Therefore, the following milestones were planned. This plan does not
remove SafeMode module immediately.

• RuboCop 0.75 ... Presents an alternative API via ConfigObsolete module
• RuboCop 0.76 ... Remove SafeMode

This is probably a better way, as it is best to offer an alternative API
in the 3rd party module.

RuboCop 0.76 has been released. This PR removes SafeMode module.

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Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

Thanks!