[Fix #8372] Fix Lint/RedundantCopEnableDirective autocorrection not removing empty # rubocop:enable comments

[dvandersluis]

Second half of the fix for #8372.

Lint/RedundantCopEnableDirective now determines what directive comments are no longer necessary and removes them outright (ie. when all the cops being enabled are redundant).

The CommentConfig class had quite a bit going on in it so I extracted a new class I called RuboCop::DirectiveComment to encapsulate some logic about determining what cops are present in a directive comment, which Lint/RedundantCopEnableDirective uses to determine if the entire comment should be removed. There probably is more refactoring that could be done here, but I kept it simple for now.

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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!