Remove unused and now obsolete macro.

* subversion/libsvn_ra_serf/ra_serf.h
  (UNUSED_CTX): Remove macro, use SVN_UNUSED now instead.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1616269 13f79535-47bb-0310-9956-ffa450edef68