parse.y: no duplicate attrset ID

* parse.y (intern_str): dynamic attrset ID is registered by
  `rb_id_attrset()` already, so no further registration is needed.
  [ruby-core:62861]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46289 b2dd03c8-39d4-4d8f-98ff-823fe69b080e