Remove unused require

`listen` uses the original thread class(listen/thread.rb) and
don't use the standard libraries one.

Author: y-yagi

lib/listen/event/loop.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'thread'
-
 require 'timeout'
 require 'listen/event/processor'
 require 'listen/thread'

lib/listen/event/queue.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'thread'
-
 require 'forwardable'
 
 module Listen

lib/listen/fsm.rb

@@ -2,8 +2,6 @@
 
 # Code copied from https://github.com/celluloid/celluloid-fsm
 
-require 'thread'
-
 module Listen
   module FSM
     # Included hook to extend class methods

lib/listen/record.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'thread'
 require 'listen/record/entry'
 require 'listen/record/symlink_detector'