Remove --parallel option from internal_investigation Rake task

Parallel static analysis is enabled by default since RuboCop 1.19.0.

Author: koic

Rakefile

@@ -34,12 +34,7 @@ task :coverage do
 end
 
 desc 'Run RuboCop over itself'
-RuboCop::RakeTask.new(:internal_investigation).tap do |task|
-  if RUBY_ENGINE == 'ruby' &&
-     RbConfig::CONFIG['host_os'] !~ /mswin|msys|mingw|cygwin|bccwin|wince|emc/
-    task.options = %w[--parallel]
-  end
-end
+RuboCop::RakeTask.new(:internal_investigation)
 
 task default: %i[
   documentation_syntax_check