error.c: remove NameError#to_s

* error.c (name_err_to_s): remove no longer needed overriding, since
  r30455 which made exc_to_s almost same.  Fixes [GH-413].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43447 b2dd03c8-39d4-4d8f-98ff-823fe69b080e