Fix an error when running rake new_cop

Follow up to rubocop/rubocop#10109.

This commit fixes the following error.

```console
% bundle exec rake new_cop\[Department/CustomCop\]
rake aborted!
ArgumentError: wrong number of arguments (given 2, expected 1)
/Users/koic/src/github.com/rubocop/rubocop-performance/Rakefile:58:in
`new'
/Users/koic/src/github.com/rubocop/rubocop-performance/Rakefile:58:in
`block in <top (required)>'
/Users/koic/.rbenv/versions/3.1.0-dev/bin/bundle:23:in `load'
/Users/koic/.rbenv/versions/3.1.0-dev/bin/bundle:23:in `<main>'
Tasks: TOP => new_cop
(See full trace by running task with --trace)
```

Rakefile

@@ -52,10 +52,7 @@ task :new_cop, [:cop] do |_task, args|
     exit!
   end
 
-  github_user = `git config github.user`.chop
-  github_user = 'your_id' if github_user.empty?
-
-  generator = RuboCop::Cop::Generator.new(cop_name, github_user)
+  generator = RuboCop::Cop::Generator.new(cop_name)
 
   generator.write_source
   generator.write_spec