[ruby/psych] Remove unused array

https://github.com/ruby/psych/commit/b9e7b4a4a4

ext/psych/lib/psych/visitors/yaml_tree.rb

@@ -17,12 +17,10 @@ module Psych
         def initialize
           @obj_to_id   = {}.compare_by_identity
           @obj_to_node = {}.compare_by_identity
-          @targets     = []
           @counter     = 0
         end
 
         def register target, node
-          @targets << target
           @obj_to_node[target] = node
         end