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