Remove __thread__ method

It's not used anywhere.

Author: jimweirich

lib/rake/thread_pool.rb

@@ -152,10 +152,6 @@ def stat(event, data=nil) # :nodoc:
     def __queue__ # :nodoc:
       @queue
     end
-
-    def __threads__ # :nodoc:
-      @threads.dup
-    end
   end
 
 end