Remove a typedef which I added in r1618906 but which isn't going to be
used very soon after all.

* subversion/include/svn_wc.h
  (svn_wc_conflict_description_private_t): Remove.
  (svn_wc_conflict_description2_t): Remove 'priv' field, and document
   how to extend this structure.



git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1619080 13f79535-47bb-0310-9956-ffa450edef68