[Fixes #6939] Remove warn for rubocop-performance

[koic]

Fixes #6939.
Fixes #6917.

This warning should not be displayed if users don't use Performance cops as follows:

• [#5977] Warn for Performance Cops #6845 (comment)
• [Fix #6917] Bump Bundler dependency to >= 1.15.0 #6919 (comment)

In the future, it will be better to replace it with a mechanism that displays a warning only when the user need Performance cops.

This PR removes a warning for rubocop-performance when runinng rubocop command.

It just leave a warning just to gem install rubocop.
https://github.com/rubocop-hq/rubocop/blob/v0.67.2/rubocop.gemspec#L47

---

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!