Remove unnecessary code to suppress warnings

Rails cops has been removed from RuboCop 0.72.
https://github.com/rubocop-hq/rubocop/releases/tag/v0.72.0

As a result of verification, the code to guard a warning is not
necessary already.

Related PR ... rubocop/rubocop-rails#83

lib/rubocop/cop/performance_cops.rb

@@ -1,13 +1,5 @@
 # frozen_string_literal: true
 
-module RuboCop
-  # RuboCop included the performance cops directly before version 1.0.0.
-  # We can remove them to avoid warnings about redefining constants.
-  module Cop
-    remove_const('Performance') if const_defined?('Performance')
-  end
-end
-
 require_relative 'performance/caller'
 require_relative 'performance/case_when_splat'
 require_relative 'performance/casecmp'