Suppress RuboCop offense

Follow rubocop/rubocop#9568.

This PR suppresses the following offense.

```console
% bundle exec rake
(snip)

Offenses:

rubocop-performance.gemspec:28:5: W: [Correctable]
Lint/RedundantCopDisableDirective: Unnecessary disabling of
Layout/LineLength.
# rubocop:disable Layout/LineLength
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

104 files inspected, 1 offense detected, 1 offense auto-correctable
RuboCop failed!
```

rubocop-performance.gemspec

@@ -25,9 +25,7 @@ Gem::Specification.new do |s|
     'homepage_uri' => 'https://docs.rubocop.org/rubocop-performance/',
     'changelog_uri' => 'https://github.com/rubocop/rubocop-performance/blob/master/CHANGELOG.md',
     'source_code_uri' => 'https://github.com/rubocop/rubocop-performance/',
-    # rubocop:disable Layout/LineLength
     'documentation_uri' => "https://docs.rubocop.org/rubocop-performance/#{RuboCop::Performance::Version.document_version}/",
-    # rubocop:enable Layout/LineLength
     'bug_tracker_uri' => 'https://github.com/rubocop/rubocop-performance/issues'
   }