eval.c: obsolete comment [ci skip]

* eval.c (rb_ensure): remove obsolete prot_tag comment.  patch by
  Jack Danger at [ruby-core:66238].  [misc #10502]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48398 b2dd03c8-39d4-4d8f-98ff-823fe69b080e