st.c: remove equality checks

* st.c (st_update): remove equality checks, callers should ensure
  the equality, otherwise the behavior is undefined.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46723 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-07-07

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46724 b2dd03c8-39d4-4d8f-98ff-823fe69b080e