Remove the warning about rubocop-performance from non-related output formats

[AlexWayfer]

Fix AtomLinter/linter-rubocop#309 after #6845

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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]

I don't quite get the fix. Why move this to one particular output format? That'd be almost the same as removing it completely.

[AlexWayfer]

I don't quite get the fix.

Did you read the commit message (PR description)? There is the issue with tools which rely on RuboCop JSON (or another standard) output. The current output in JSON format (--format=json) is invalid JSON. I think, the same for HTML or other (quiet?) formats. Is this not enough? Have tools to bypass any non-formatted text somehow?

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New conflicts resolved.

[bbatsov]

Why are those tools reading something from stderr? That seems pretty weird.

[AlexWayfer]

Why are those tools reading something from stderr? That seems pretty weird.

Let me try to discuss this in the tool's issue. Thank you for advice.

[AlexWayfer]

I think, we can solve it in tools. Thank you, sorry to bother you.