Remove duplicated ensure_open call.

Author: itarato

src/main/ruby/truffleruby/core/truffle/dir_operations.rb

@@ -60,7 +60,6 @@ def self.readdir_multiple(dir, resolve_type, exclude_self_and_parent, entries)
     end
 
     def self.readdir_name(dir)
-      dir.__send__(:ensure_open)
       dirptr = Primitive.object_ivar_get(dir, :@ptr)
       entry = Truffle::POSIX.truffleposix_readdir_name(dirptr)
       Errno.handle unless entry