Fix multi require concurrent/executor/cached_thread_pool

Author: OuYangJinTing

lib/concurrent-ruby/concurrent/executors.rb

@@ -10,7 +10,6 @@
 require 'concurrent/executor/ruby_executor_service'
 require 'concurrent/executor/ruby_single_thread_executor'
 require 'concurrent/executor/ruby_thread_pool_executor'
-require 'concurrent/executor/cached_thread_pool'
 require 'concurrent/executor/safe_task_executor'
 require 'concurrent/executor/serial_executor_service'
 require 'concurrent/executor/serialized_execution'